subscriber: document time's --cfg unsound_local_offset

[hawkw]

Motivation

The time crate must be compiled with --cfg unsound_local_offset in
order for local timestamps to be enabled. For users whose first exposure
to the time crate's API is via tracing-subscriber's time timestamp
formatters, this is potentially very surprising! Therefore, although
this cfg is not part of tracing's API surface, we should probably
document this aspect of time's API in the
tracing-subscriber::fmt::time documentation.

Solution

This branch adds warnings in the time::LocalTime type's API docs, in
the struct-level documentation and on the new constructor, describing
that --cfg unsound_local_offset is necessary to record local
timestamps and referring users to the time documentation. I also added
unsound_local_offset to the doc(cfg(...)) attributes for the
LocalTime type.

While I was changing tracing-subscriber's docs, I also fixed a couple
formatting issues I noticed.

Fixes #1688

[lilyball]

This looks great, thanks! My only quibble is that the cfg flag is only necessary on unix and is not necessary on Windows.