Update the rewritten documents for the fixes merged into next

The documents, the example config and the help text are rebased onto the
fourteen fixes on next. The sentences those fixes made false now match the
code: an unknown config key, a relative index_path and a value that is not a
size are refused when the config loads; a backup saves the destination in the
local index only after it writes the marker file, and no other command saves
it; a backup skips the local index, its side files and the lock file; a binary
from go install reports its tag and counts as a release; a declined deletion
prompt exits 1 with "deletion not confirmed"; the S3 keys are set before an
s3:// storage_url. No document quoted the old messages for an unknown snapshot
ID, a wrong private key or an unreadable blob.

Model: fable-5-1

configs/config.example.yml

@@ -320,6 +320,7 @@ storage_url: "rclone://myremote/path/to/backups"
 # Path to local SQLite index database
 # This database tracks file state for incremental backups
 # The VAULTIK_INDEX_PATH environment variable overrides this setting
+# Both must be absolute paths; a leading ~ is expanded
 # Default: the platform data directory, e.g.
 #   macOS: ~/Library/Application Support/vaultik/index.sqlite
 #   Linux: ~/.local/share/vaultik/index.sqlite