Read all five documents against the code and fix what the sixth review found

Fixes the four findings of the sixth review: `snapshot remove` with
`--local-only`, the two comma-chained permission lists, six undefined terms,
and the sentence about an unfinished snapshot in the local index.

Then reads every sentence of `README.md`, `ARCHITECTURE.md`,
`docs/DATAMODEL.md`, `docs/REPOSTRUCTURE.md` and the comments in
`config.example.yml` against the code for four faults: a command's needs or
behaviour stated without regard to a flag, a false absolute, three or more
things chained with commas, and a technical word not defined on first use.
Each sentence changed is listed on the pull request.

The help string of `snapshot remove --dry-run` now says it needs no
permission to delete.

Model: fable-5-1

config.example.yml

@@ -3,7 +3,7 @@
 # defaults; where an option has a default, its comment gives it.
 # Copy this file and change the values you need
 
-# Age recipient public keys for encryption
+# Age recipient public keys for encryption (age: https://age-encryption.org/)
 # Backups are encrypted to these public keys. snapshot create needs at least
 # one; listing, verifying and restoring do not
 # Generate a key pair, and show its public key, with:
@@ -11,6 +11,7 @@
 #   grep 'public key' vaultik_backup_private_key.txt
 # Keep the private key file: restoring needs it. Do not pipe age-keygen
 # straight into grep, which would discard the private key
+# Default: none
 age_recipients:
   - age1cj2k2addawy294f6k2gr2mf9gps9r3syplryxca3nvxj3daqm96qfp84tz
 
@@ -268,6 +269,7 @@ snapshots:
       - "**/vendor"
 
 # Global patterns to exclude from all backups
+# Default: none
 exclude:
   - "*.tmp"
 
@@ -276,8 +278,8 @@ exclude:
 # An s3:// URL also needs s3.access_key_id and s3.secret_access_key below
 # A file:// path must be absolute, and the directory must already exist:
 # vaultik never creates it
-# An rclone:// URL needs the remote to be defined in rclone's own config
-# (rclone config)
+# An rclone:// URL names a remote of rclone (https://rclone.org/); the remote
+# must be defined in rclone's own config (rclone config)
 storage_url: "rclone://myremote/path/to/backups"
 
 # S3-compatible storage configuration
@@ -310,7 +312,7 @@ storage_url: "rclone://myremote/path/to/backups"
 #
 #  # Part size for multipart uploads
 #  # Minimum 5MiB, maximum 5GiB; affects memory usage during upload
-#  # A blob too large for 10,000 parts of this size gets larger parts
+#  # An object too large for 10,000 parts of this size gets larger parts
 #  # Supports: 10MB, 16MiB, 100MiB, etc. (5MB is below the minimum)
 #  # Default: 5MiB
 #  #part_size: 5MiB
@@ -323,8 +325,9 @@ storage_url: "rclone://myremote/path/to/backups"
 #   Linux: ~/.local/share/vaultik/index.sqlite
 #index_path: /path/to/index.sqlite
 
-# Average chunk size for content-defined chunking
-# Smaller chunks = better deduplication but more metadata
+# Average size of a chunk, a piece of a file's content. Chunk boundaries
+# depend on the file's content, not on fixed offsets
+# Smaller chunks find more duplicate content and make the index larger
 # Minimum 1MB
 # Supports: 10MB, 5M, 1GB, 64MiB, etc.
 # Default: 10MB
@@ -334,9 +337,10 @@ storage_url: "rclone://myremote/path/to/backups"
 # Multiple chunks are packed into blobs up to this size
 # Must be at least four times chunk_size (the largest chunk the chunker can
 # emit); a smaller limit would let a single-chunk blob exceed it.
-# Chunking uses no secret (the FastCDC parameters are fixed and public). At a
-# large limit a blob holds hundreds of chunks, so individual chunk lengths are
-# not visible in its size; lowering the limit toward chunk_size exposes them.
+# Chunking uses no secret (the parameters of the FastCDC chunking algorithm
+# are fixed and public). At a large limit a blob holds hundreds of chunks, so
+# individual chunk lengths are not visible in its size; lowering the limit
+# toward chunk_size exposes them.
 # A backup needs free temporary space, because each blob is written whole to
 # a temporary file before it is uploaded (up to about blob_size_limit; an
 # rclone destination that cannot stream uploads needs about twice that) and
@@ -352,12 +356,14 @@ storage_url: "rclone://myremote/path/to/backups"
 # Default: 3
 compression_level: 5
 
-# How old a blob that no snapshot lists must be before prune deletes it, and
-# how old every object in a directory under metadata/ with no manifest must
-# be before prune deletes that directory
-# A backup uploads its blobs and its db.zst.age before its manifest, so on a
-# destination shared between hosts or users this must be longer than the
-# longest backup run
+# How old a blob that no snapshot lists must be before vaultik prune deletes
+# it. Each snapshot has a directory under metadata/ at the destination, and
+# its manifest there lists the blobs it uses. This is also how old every
+# object in a directory with no manifest must be before vaultik prune deletes
+# that directory
+# A backup uploads its blobs and its encrypted database (db.zst.age) before
+# its manifest, so on a destination shared between hosts or users this must
+# be longer than the longest backup run
 # It does not protect a blob a backup reuses from an earlier run that stopped
 # before its manifest: that blob keeps the time of its first upload, so a
 # prune can delete it