pw_thread: Improve docs for thread backends
pw_thread: Improve docs for thread backends

Improves the verbiage in the documentation for thread backends
and that they may use native detached threads if and only if
PW_THREAD_JOINING_ENABLED is set to false.

Change-Id: I0d08c8a630ed64b81c585fe701510beaf3f5bcbc
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/110457
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>
Reviewed-by: Leonard Zgrablic <c23291568ca4a15010570a639090ed5506ef9143@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

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