Fix bug fdo#80431 Help entries for GAMMA functions

Provides help entries for functions GAMMA.DIST GAMMA.INV and GAMMALN.PRECISE

Change-Id: I9a675d40f7eca6a799e49706e062ea77e05feffd
Reviewed-on: https://gerrit.libreoffice.org/9916
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>