Bug 16365: [QA Follow-up] Add some comment lines to Cache.pm

Bug 16365: [QA Follow-up] Add some comment lines to Cache.pm

Resolve typo inifinite too.
Adds a few lines in order to stress that the thawed key of the L1
cache SHOULD ONLY be used for unsafe calls, and not be mixed with
regular (safe) calls.

Test plan:
Nothing to test, but verify the quality of the added comments.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>