perlreftut: lots of small changes

perlreftut: lots of small changes

- use I<...> for italics/emphasis, not '...'
- "Make Rule 2" was missing its "=head3" that marks it as a section
- consistently use 2 spaces for indentation
- add internal hyperlinks to the Make/Use Rules
- consistently use C<...> to refer to variables
- consistently use 'for' (not 'foreach')
- hyperlink push/ref to their documentation in perlfunc
- hyperlink ==/eq to their documentation in perlop
- hyperlink "use strict 'refs'" to strict
- remove mention of "soft reference" because according to perlref that
  term has been officially discouraged since 1996