PM: runtime: Add documentation for pm_runtime_resume_and_get()

Commit dd8088d5 ("PM: runtime: Add pm_runtime_resume_and_get to
deal with usage counter") added a new runtime-PM API function without
updating the documentation in runtime_pm.rst.

Add the missing documentation.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: dd8088d5 ("PM: runtime: Add pm_runtime_resume_and_get to deal with usage counter")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>