perlretut: Cleanup, nits

This adds some C<>, I<>, changes non-literal text from C<> to I<>.
It changes some phrases that are enclosed in single quotes to the more
idiomatic double quotes.

It standardizes on single characters within C<> to be C<'x'>.  This is
not standardized in our documentation, and people change it back and
forth.  I prefer the extra quotes, as it otherwise blends in to the
background on html displays.

It converts the few 'regex' terms to 'regexp'.

It fixes some numbered lists to display not so uglily

It removes the cautions about the features that are no longer experimental

It corrects some grammar

Author: khwilliamson