Documentation for std::fs::set_permissions doesn't cover symlink behavior

[sunfishcode]

The documentation for std::fs::set_permissions doesn't say what it does if the given path names a symlink.

Experimentally, it seems that on Unix-family platforms it follows symlinks, while on Windows it does not. Though surprising, it's likely just a result of using the chmod and SetFileAttributes functions on their respective platform. The documentation does say that use of those functions isn't guaranteed, so it'd be useful to document what guarantees are intended, if any.