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

@@ -22,7 +22,7 @@ You can either put this in a script (`script.vim`) and run it (`:source script.v
 ```vim
 function! SuperTab()
   let l:part = strpart(getline('.'),col('.')-2,1)
-  if (l:part=~'^\W\?$')
+  if (l:part =~ '^\W\?$')
       return "\<Tab>"
   else
       return "\<C-n>"
@@ -316,10 +316,13 @@ Checks if it's the same instance object.
 ### Regexp matches
 
 ```vim
-"hello" =~ '/x/'
-"hello" !~ '/x/'
+"hello" =~ 'xx*'
+"hello" !~ 'xx*'
+"hello" =~ '\v<\d+>'
 ```
 
+`\v` enables "extended" regex mode which allows word boundary (`<>`), `+`, and more.
+
 ### Single line
 
 ```vim