doc string of next-single-char-property-change

[Drew Adams]

The doc string is missing this important info that is in the Elisp
manual (node Property Search): what value is returned if no such
property change is found. The manual says it succinctly:

  if no change is found before the end of the OBJECT, it returns the
  maximum valid position in OBJECT

This is all the more important as this behavior is different from
that of `next-single-property-change'.


In GNU Emacs 22.1.50.1 (i386-mingw-nt5.1.2600)
 of 2007-05-22 on LENNART-69DE564
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (3.4) --cflags -Ic:/g/include'