[perl #116773] Documentation of $ in perlre

I have applied this change along with a rewording of /m's description.

I wasn't sure where we might like to add the "equivalent definitions," so I didn't add them.  If 
someone has a good idea, it sounded plausible.

-- 
rjbs

• [perl #116773] Documentation of $ in perlre by Ricardo SIGNES via RT