Changed docs to be less misleading in path::file_name(&self)

[Andyson007]

The documentation of Path::file_name(&self) says that it will return None iff it ends with ... This does not match the behavior in cases such as

• "."
• "/"

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) some time within the next two weeks.

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[ChrisDenton]

The cases that can lead to None are, after normalization:

• ""
• ends with ..
• it contains only a root dir (i.e. "/") or (on Windows) a prefix (e.g. "C:")

I say "after normalization" because ., ./., ././., etc are all normalized to the empty string.

[alex-semenyuk]

Triage: comment is not addressed
@rustbot label: +S-waiting-on-author, -S-waiting-on-review

[alex-semenyuk]

@Andyson007
From wg-triage. Closed this PR due to inactivity. Feel free to reopen or raised new one. Thanks for your efforts.