Add POD documentation to perli11ndoc to fix

Closed Avery Adams requested to merge oldtechaa/perl-glib-object-introspection:master into master

I opened CPAN issue #125154 to indicate the need for POD documentation for perli11ndoc. Debian, at least, requires documentation for all binaries shipped in $PATH. This adds some basic POD documentation that can be converted to nroff in Debian.

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