docs(lock): clarify ILockingProvider contract and lock behavior

[joshtrichards]

• Resolves: #

Summary

Clarify the public ILockingProvider documentation to better describe its role in Nextcloud's locking architecture.

Changes

• Clarify that the provider is a low-level primitive primarily used by storage implementations and indirectly by the Files Node and View APIs
• Document shared and exclusive lock behavior
• Distinguish it from the VFS-level Node/View locking APIs used during normal file operations
• Distinguish it from the OCP\Files\Lock APIs used for user, application, and collaborative-editor file locks (i.e. via the files_app)
• Document that it can also coordinate access to non-file server resources; for example, the encryption app uses it to prevent concurrent key-generation operations
• Add method-level descriptions, clean up docblock parameter typing, and improve existing wording

TODO

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[CarlSchwan]

while at it you might want to add the Implementable attribute here