PPC: Document pattern of symbols reserved for Perl in XS code

XS writers don't know what symbols to avoid creating for their own purposes.

Thousands of undocumented Perl symbols are exposed to XS code.  Many C 
symbols are as well.  However, C furnishes a template giving their 
syntax.  Basically, anything matching the template is a reserved word 
for the C implementation.

I am proposing that we do the same, and the one regen/embed.pl currently 
is using is

Any symbol beginning with "PL_"
Any symbol containing perl, Perl, or PERL, usually delimitted on
     both sides so as to keep it from being part of a larger word.

POSIX feels free to add new functions that could possibly collide with 
ones in existing code.  'atomic_int', for example in Posix 2024.  I 
think we can do the same.

Many symbols we've created end in an underscore, as an undocumented 
attempt to indicate that these are not intended for public use.  Tony 
Cook has pointed out that other projects that use Perl do the same.  I'm 
proposing that we thus restrict ourselves further for new symbols to 
ending in

  m/ _ [[:alnum:]]* pl_ \z /x

And document this.