Document #293 in local-offset feature description

[jcdyer]

This PR adds documentation to the feature flag for local-offset indicating that the feature is useless on unix platforms, with a reference to #293.

I just spent longer than I wanted trying to figure out why OffsetDateTime::try_local_now was returning IndeterminateDateTime. The path I had to take to figure it out:

1. Go to the documentation for OffsetDateTime::try_now_local. No mention of the issue.
2. Check the source for that method. No mention of the issue.
3. Go to the documentation for UtcOffset::local_offset_at. No mention.
4. Find the undocumented private function try_local_offset_at. Reference to The call to localtime_r may be unsound #293.
5. Check github for The call to localtime_r may be unsound #293. Issue is marked resolved.
6. Switch dependency to main. Discover feature flag and new method names. Error persists.
7. Repeat 1-4 with new nomenclature. Same reference.

Hopefully a mention on the documentation for the feature flag will prevent others from having to follow that same path.

[jcdyer]

I'd be happy to also add documentation to the same effect directly on all the #[cfg(feature = "local-offset")] methods.

[dekellum]

See also #296. IMO, thats the one that shouldn't have been closed.

[jhpratt]

To be clear: #293 is technically resolved. The call is no longer unsound, and will no longer randomly crash your program. #296 was more of a question, and was closed accordingly.

In will look at this PR later today. Adding a note in documentation has been planned; I've just been preoccupied.

[jcdyer]

I can update this to point to 296. The comment in the code mentions 293, so I followed suit.

[jhpratt]

Looks like I already added the note to documentation in 288c942. Unless there's something I missed, I believe this should be closed. The feature flag itself shouldn't need to be documented as such, as it's the individual methods that people will be looking at primarily.