perlre clean-up

perlre clean-up

Mostly typos, grammatical errors and factual errors (mostly due to
bitrot), but also:

• The section explaining how to work around the lack of look behind
  obviously has not been relevant for years. :-)
• Since we have relative backreferences, we might as well use them in
  the explanation of the (?>...) construct.
• Note that it’s possible to backtrack *past* (?>...), but
  not into it.
• (?:non-zero-length|zero-length)* is *not* equivalent to nzl*|zl? as
 "aaaaab" =~ /(?:a|(?{print "hello"})(?=(b)))*/ demonstrates.
• The custom re engine section doesn’t mention custom re engines. :-)