PM: runtime: document common mistake with pm_runtime_get_sync()

pm_runtime_get_sync(), contradictory to intuition, does not drop the
runtime PM usage counter on errors which lead to several wrong usages in
drivers (missing the put).  pm_runtime_resume_and_get() was added as a
better implementation so document the preference of using it, hoping it
will stop bad patterns.

Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
[ rjw: Documentation change edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>