vimscript: Function stridx return value is an index (#988)

* Bugfix: function return value is an index

* White-space change

* Bugfix and clarification

vimscript-functions.md

@@ -152,7 +152,7 @@ Functions
     substitute(str, '.', 'x', 'g')
     strpart("abcdef", 3, 2)    " == "de" (substring)
     strpart("abcdef", 3)       " == "def"
-    stridx("abcdef", "e")      " == "e"
+    stridx("abcdef", "e")      " == 4
     strridx()                  " reverse
 
     matchstr('testing','test')  " == 'test' (or '')

vimscript.md

@@ -316,10 +316,13 @@ Checks if it's the same instance object.
 ### Regexp matches
 
 ```vim
-"hello" =~ '/x/'
+"hello" =~ 'xx*'
-"hello" !~ '/x/'
+"hello" !~ 'xx*'
+"hello" =~ '\v<\d+>'
 ```
 
+`\v` enables "extended" regex mode which allows word boundary (`<>`), `+`, and more.
+
 ### Single line
 
 ```vim