More precise 'regexp-opt' documentation

* lisp/emacs-lisp/regexp-opt.el (regexp-opt):
* doc/lispref/searching.texi (Regexp Functions):
Be more specific about how the KEEP-ORDER argument actually works.
If nil, the regexp guarantees a longest match; this is the behaviour
that many callers implicitly rely on.

Author: mattiase

doc/lispref/searching.texi

@@ -1780,11 +1780,11 @@ if it is necessary to ensure that a postfix operator appended to
 it will apply to the whole expression.
 @end table
 
-The optional argument @var{keep-order}, if @code{nil} or omitted,
-allows the returned regexp to match the strings in any order.  If
-non-@code{nil}, the match is guaranteed to be performed in the order
-given, as if the strings were made into a regexp by joining them with
-the @samp{\|} operator.
+The optional argument @var{keep-order}, if non-@code{nil}, forces the
+match to be performed in the order given, as if the strings were made
+into a regexp by joining them with the @samp{\|} operator.  If nil or
+omitted, the returned regexp will always match the longest string
+possible.
 
 Up to reordering, the resulting regexp of @code{regexp-opt} is
 equivalent to but usually more efficient than that of a simplified

lisp/emacs-lisp/regexp-opt.el

@@ -114,11 +114,11 @@ nil
     necessary to ensure that a postfix operator appended to it will
     apply to the whole expression.
 
-The optional argument KEEP-ORDER, if nil or omitted, allows the
-returned regexp to match the strings in any order.  If non-nil,
-the match is guaranteed to be performed in the order given, as if
-the strings were made into a regexp by joining them with the
-`\\|' operator.
+The optional argument KEEP-ORDER, if non-nil, forces the match to
+be performed in the order given, as if the strings were made into
+a regexp by joining them with the `\\|' operator.  If nil or
+omitted, the returned regexp is will always match the longest
+string possible.
 
 Up to reordering, the resulting regexp is equivalent to but
 usually more efficient than that of a simplified version: