string-search robustness and documentation improvement (bug#43598)

* src/fns.c (Fstring_search): Check START-POS argument range.
Simplify logic.  Improve doc string.
* test/src/fns-tests.el (string-search): Add test cases.
* doc/lispref/strings.texi (Text Comparison): Elaborate.
* lisp/emacs-lisp/byte-opt.el (pure-fns): Mark string-search as pure.

lisp/emacs-lisp/byte-opt.el

@@ -1264,6 +1264,7 @@
          floor ceiling round truncate
          ffloor fceiling fround ftruncate
          string= string-equal string< string-lessp
+         string-search
          consp atom listp nlistp propert-list-p
          sequencep arrayp vectorp stringp bool-vector-p hash-table-p
          null not