"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.")