[5.1] add doc / internal / coding standard

[Andrew Cagney]

FYI,

I'm adding a review of this section to the 5.1 release criteria  Given
that the second paragraph reads:

     GDB's policy on the use of prototypes is that prototypes are used
to
  *declare* functions but never to *define* them.  Simple macros are
used
  in the declarations, so that a non-ANSI compiler can compile GDB
  without trouble.  The simple macro calls are used like this:

I've got to do something :-)

	Andrew