Hyperlink symbol names without word syntax in Help

* lisp/emacs-lisp/lisp-mode.el (lisp-el-font-lock-keywords-2)
(lisp-cl-font-lock-keywords-2): Allow single-character symbol names.
* lisp/help-mode.el (help-xref-symbol-regexp): Also match symbol
names starting with symbol syntax (bug#6601, bug#24309).
* test/lisp/help-mode-tests.el (help-mode-tests-xref-button): Test
hyperlink creation for function names without symbol syntax.

lisp/help-mode.el

@@ -357,8 +357,7 @@ Commands:
  		    "\\(symbol\\|program\\|property\\)\\|" ; Don't link
 		    "\\(source \\(?:code \\)?\\(?:of\\|for\\)\\)\\)"
 		    "[ \t\n]+\\)?"
-		    ;; Note starting with word-syntax character:
-		    "['`‘]\\(\\sw\\(\\sw\\|\\s_\\)+\\|`\\)['’]"))
+                    "['`‘]\\(\\(?:\\sw\\|\\s_\\)+\\|`\\)['’]"))
   "Regexp matching doc string references to symbols.
 
 The words preceding the quoted symbol can be used in doc strings to