msm: msm_bus: Update docstrings of a few APIs

The docstrings of some of the MSM bus client APIs have fallen
out-of-date.  Update them to reflect the current implementation.

Change-Id: I7e8ae939e30bdcd35e9389327e0882f3e679218b
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>