RFC: Doc patch to better explain /dual regex modifiers

I have heard that people are having a hard time coping with these 
modifiers.  In documenting them, I lost sight of the original purpose of 
the three /dul ones, which was so that regexes could be serialized and 
later reconstituted exactly; and that they are selected automatically by 
various pragmas, so that they should rarely be used explicitly.

I also have changed the documented name of the /a modifier to 
ASCII-restrict and/or ASCII-safe to better correspond with its purpose. 
  I'm open to better names.

I didn't know if it was better to push this and invite further patches, 
or do an RFC.  Since I won't be at a computer much in the next few 
weeks, I decided to go the RFC route; not sure why that seemed better. 
I've looked at this patch dozens of times now, and each time I see a few 
changes to make, but am not seeing a whole new better way to do the 
explanation.

• RFC: Doc patch to better explain /dual regex modifiers by Karl Williamson