Unicode::UCD: Correct wrong pod info

 This was erroneous.  Extra clarifications are also added.
Unicode::UCD: Move function in file.

This is in preparation for making this function public, and it should be
listed in the pod later than it otherwise would be.
Unicode::UCD Clarifications in pod

There are no "missing" values in inversion maps; there is a default
value returned for each one.  So change the example variables' names.
Plus another sentence rewording for clarity.
Make Unicode::UCD::search_invlist() available

This commit documents this function, removing the initial underscore
from its name.  (And it hardens input checking.)