include: uart: Another pass on improving docstrings

1. Avoid outdated references to registers of a particular hardware
in the generic API.

2. Propagate specifications/clarifications of ISR behavior to
docstrings of more functions which guaranteed to work only in ISR.

This continues work previously done in:

38f78e80cf650133827b80cad1c6abbba13d56b5
0fdc9b5b125c60d80a368360851a50f1f03710aa
etc.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>