docs(std): add error docs for path canonicalize

[smirzaei]

This PR adds the missing error documentation for both Path.canonicalize and PathBuf.canonicalize methods. Since both methods are wappers around fs::canonicalize, the error doc is copied directly from that function.

This makes it faster to find what errors might arise when calling path.canonicalize or path_buf.canonicalize in the editor itself without needing to drill down to the fs::canonicalzie docs.

[rustbot]

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