Bug 11336: (follow-up) improve POD for _FixPriority()

Bug 11336: (follow-up) improve POD for _FixPriority()

This patch improves the POD for C4::Reserves::_FixPriority()
to (hopefully) describe its function thoroughly.  It also
adjusts the call of _FixPriority() by CancelReserve() to
omit passing reserve_id, since by that point no row in
the reserves table for that request still exists.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 543e1dc6731d4a26b777223920bfe4037e7546dd)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>