diff --git a/lisp/isearch.el b/lisp/isearch.el
index 48354d39d48..5257a23223f 100644
--- a/lisp/isearch.el
+++ b/lisp/isearch.el
@@ -2665,8 +2665,9 @@ the word mode."
   "Non-default value overrides the behavior of `isearch-search-fun-default'.
 This variable's value should be a function, which will be called
 with no arguments, and should return a function that takes three
-arguments: STRING, BOUND, and NOERROR.  See `re-search-forward'
-for the meaning of BOUND and NOERROR arguments.
+arguments: STRING, BOUND, and NOERROR.  STRING is the string to
+be searched for.  See `re-search-forward' for the meaning of
+BOUND and NOERROR arguments.
 
 This returned function will be used by `isearch-search-string' to
 search for the first occurrence of STRING.")