Bug 17935: Adjust some POD lines, fix a few typos

Bug 17935: Adjust some POD lines, fix a few typos

This patch does the following:

[1] Move some POD lines from Cache to Caches.
[2] Correct C4::Plugins to Koha::Plugins in POD line of Koha::Plugins
[3] POD Koha/AuthorisedValue.pm: lib_opac moved to opac_description
[4] The POD in Koha/Patron.pm uses head2 and head3 inconsistently.
    Ran s/^=head2/=head3/ on those lines (7 substitutions on 7 lines)
[5] Correct a copied POD line from reports/issues_stats.pl in
    reports/reserve_stats.pl.
[6] Correct a test description in t/db_dependent/Koha/Authorities.t.
    You should never delete the library :)
[7] Correct typo shouild in a comment of rebuild_zebra.pl

Test plan:
[1] Read the patch. Does it make sense?
[2] Run perldoc Koha/Cache.pm and Koha/Caches.pm
[3] Run t/db_dependent/Koha/Authorities.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Cook <dcook@prosentient.com.au>

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>