MINOR: doc: missing entries for sc-add-gpc()

MINOR: doc: missing entries for sc-add-gpc()

When sc-add-gpc() action was implemented in 5a72d03 ("MINOR:
stick-table: implement the sc-add-gpc() action"), its usage was
only documented for "http-request", but according to the code it
now applies everywhere sc-inc-gpc() is mentioned.

Adding the missing entries in the doc everywhere the action may
be used.

The issue was detected by the haproxy-controller bot and was reported
by Pratik Mohanty and Marko Juraga.

No backport needed, unless 5a72d03 ("MINOR: stick-table: implement the
sc-add-gpc() action") is being backported.