bug#44141: 26.3; doc of `default-boundp' and "void" variable values

[Drew Adams]

The doc of `default-boundp' could be improved by saying explicitly what
it means for a symbol's default value to be "void".  This is not done
anywhere, AFAICT - "void" is undefined.  And for Lisp, especially,
there's some risk of confusion with the value `nil'.

1. The Elisp manual is pretty good on `default-boundp', but saying
something about what "void" means would help.

2. The doc string is not so helpful - you really need to read the short
paragraph in the manual to understand it, I think.  Consider beefing up
the doc string with something like what is said in the manual - it's
short and explanatory.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''