replace-match with optional fields

[Luc Teirlinck]

I do not know whether it was decided that the behavior of
`replace-match' in the described case should be changed or not.  (I do
not have a particularly strong opinion on this).  But the
documentation is ambiguous either way.  Assuming the decision was to
leave the behavior as is, I propose the following change in the docstring
and would make a similar change in the Elips manual.  (Otherwise, I
would recommend other doc fixes.)

===File ~/search.c-diff=====================================
*** search.c.~1.175.~   Tue Sep  2 08:19:06 2003
--- search.c    Fri Jan  9 15:38:35 2004
***************
*** 2179,2185 ****
  Otherwise treat `\\' as special:
    `\\&' in NEWTEXT means substitute original matched text.
    `\\N' means substitute what matched the Nth `\\(...\\)'.
!        If Nth parens didn't match, substitute nothing.
    `\\\\' means insert one `\\'.
  Case conversion does not apply to these substitutions.
  
--- 2179,2185 ----
  Otherwise treat `\\' as special:
    `\\&' in NEWTEXT means substitute original matched text.
    `\\N' means substitute what matched the Nth `\\(...\\)'.
!        If Nth parens didn't match, leave the `\\N' unsubstituted.
    `\\\\' means insert one `\\'.
  Case conversion does not apply to these substitutions.
  
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