Refactor: libcrmcommon: Add comment for PCMK_service env variable

This variable was unused when it was added in f3c4f9e, so it's not
obvious why we keep it. It was later used in fencing.py.py in
fence-agents, starting with fence-agents commit c9e4492.

Ref T637

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>