Improve documentation for file locking
Add notes to each method stating that locks get dropped on close.
Clarify the return values of the try methods: they're only defined if
the lock is held via a *different* file handle/descriptor. That goes
along with the documentation that calling them while holding a lock via
the *same* file handle/descriptor may deadlock.
Document the behavior of unlock if no lock is held.
r? `@m-ou-se`
(Documentation changes requested in https://github.com/rust-lang/rust/issues/130994 .)
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