autodoc.pl: Enhance apidoc_section feature

autodoc.pl: Enhance apidoc_section feature

This feature allows documentation destined for perlapi or perlintern to
be split into sections of related functions, no matter where the
documentation source is.  Prior to this commit the line had to contain
the exact text of the title of the section.  Now it can be a $variable
name that autodoc.pl expands to the title. It still has to be an exact
match for the variable in autodoc, but now, the expanded text can be
changed in autodoc alone, without other files needing to be updated at
the same time.