bug#20625: 25.0.50; doc of `define-error` is incorrect

[Drew Adams <drew.adams@oracle.com>]

From `C-h f':

 (define-error NAME MESSAGE &optional PARENT)

 Define NAME as a new error signal.
 MESSAGE is a string that will be output to the echo area if such an error
 is signaled without being caught by a `condition-case'.
 PARENT is either a signal or a list of signals from which it inherits.
 Defaults to `error'.

No, MESSAGE is not necessarily a string. It is a required argument, but
the code explicitly makes use of the case where it is nil.  And existing
code delivered with Emacs (e.g. bookmark.el) makes use of a nil MESSAGE.
That case should be documented.

The doc in the manual has the same bug (node Error Symbols).  MESSAGE is
*not* necessarily a string.




In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'