Bug 18570: (QA follow-up) Improved POD

Bug 18570: (QA follow-up) Improved POD

This tweaks the perldoc for SendQueuedMessages and adds
some for _get_unsent_messages.

TEST PLAN
---------

perldoc C4::Letters
-- look at _add_attachments (ugly), _get_unsent_messages (non-existent),
   SendQueuedMessages (no reference to borrowernumber or letter_code).
apply patch
perldoc C4::Letters
-- confirm that SendQueuedMessages and _get_unsent_messages
   have reasonable POD information.
-- notice how _add_attachments' POD is now readable

run koha qa test tools

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>