#2853 Docs: document mount-independent file grant sources for --grants-dir and --grants-file

swamp-club#2848 changes how grants loaded from --grants-dir and --grants-file are stored. Their source moves from file: to file:grants-dir/ and file:grants-file, so serve instances sharing a datastore stop revoking each other's grants when they mount the same files at different paths.

Pages to update in content/manual:

• reference/swamp-serve/authorization.md: the source table lists only file: (grants/ directory). Add rows for file:grants-dir/ and file:grants-file.
• how-to/swamp-serve/manage-grants-with-files.md: the note that file grants appear as file: should mention the two external forms.
• how-to/swamp-serve/load-external-grants-at-startup.md: add that instances sharing one datastore must load the same grant files. The mount path no longer matters, but an instance without the --grants-dir, or with different files, still revokes the other instance's grants from it.

Upgrade note: on first reconcile after upgrade, grants stored under the old absolute-path sources are revoked and recreated under the new names. Until every instance on a shared datastore is upgraded, older instances keep the old behaviour.

Also: access reload results now report the new source names for external files instead of the full path.