Clarify the meaning of "dynamic scoping" in perlsub

This clarifies an ambiguity regarding local. Giving
a temporary value to a global variable is known as
dynamic scoping (as opposed to the other interpretation
which is incorrect: creating a local variable is not known
as dynamic scoping)
Merge pull request #2 from doherty/perlsub-dynamic-scoping-clarity-for-perl-doc-cats

Clarify the meaning of "dynamic scoping" in perlsub

Thanks to @doherty .