clean up docs for `File::sync_*`
* Clarify that `sync_all` also writes data and not just metadata.
* Clarify that dropping a file is not equivalent to calling `sync_all` and ignoring the result. `sync_all` the still the recommended way to detect errors before closing, because we don't have a dedicated method for that.
* Add a link from `sync_all` to `sync_data`, because that's what the user might want to use instead.
* Add doc aliases for `fsync` -> `sync_all` and `fdatasync` -> `sync_data`. Those are the POSIX standard names for these functions. I was trying to find out what we call `fsync` in Rust and had to search through the source code to find it, so this alias should help with that in the future.
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