bug#14743: 24.3.50; doc string of `read-regexp'

[Drew Adams]

The name DEFAULT is used with two different meanings in the doc string
(wholePARAMETER or first element of cons-valued arg).  It seems, from
looking at the code, that that is OK, in the sense that the same
behavior applies to both meanings.

However, the doc should say what happens wrt the return value if DEFAULT
is nil and the user enters empty input.  The answer, it seems, is that
the empty string is returned.

In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-06-27 on ODIEONE
Bzr revision: 113205 dgutov@yandex.ru-20130627095155-f1lv1c7xf99g1sss
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/binary --enable-checking=yes,glyphs
 CFLAGS=-O0 -g3 LDFLAGS=-Lc:/Devel/emacs/lib
 CPPFLAGS=-Ic:/Devel/emacs/include'