Fix the ten findings of the review on the destination, the marker file and chunk sizes

README.md: the "Getting started" destination advice, the Full Disk Access
paragraph moved above the block with the reopen step, the mount-point
case, the remote nuke failure sentence, the hostname setting, the upload
order under "Sharing a destination", the confirmation list and the dev
stamp sentence. configs/config.example.yml: the storage_url and
blob_size_limit comments. docs/ARCHITECTURE.md and docs/DATAMODEL.md:
the Snapshot ID and Marker file terms, and "marker file" in place of
"destination marker".

Model: fable-5-1

configs/config.example.yml

@@ -273,7 +273,8 @@ snapshots:
 exclude:
   - "*.tmp"
 
-# Storage URL - use either this OR the s3 section below
+# Storage URL. When it is not set, the endpoint, bucket and prefix in the s3
+# section below are used instead.
 # Supports: s3://bucket/prefix, file:///path, rclone://remote/path
 # 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:
@@ -339,9 +340,10 @@ storage_url: "rclone://myremote/path/to/backups"
 # 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 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.
+# are fixed and public). A full blob at a large limit holds hundreds of
+# chunks, so its size does not show any one chunk's length. The last blob of
+# each configured path can hold a single chunk. Lowering the limit toward
+# chunk_size exposes chunk lengths.
 # 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