_StringInsert(): add Remark for clarity

[c.haslam]

As the help is now, it is not clear whether the string is inserted before or after $iPosition.

I suggest adding a Remark:

Inserts the string after $iPosition characters of $sString. For $iPositon < 0, inserts it before -$iPosition characters from the end of $sString.

This is, to me, a bit strange, but this is the way it is!

I have checked the code, and run a simple example:

#include <String.au3>
Local $t='CatSukie'
MsgBox(0,'',_StringInsert($t,'Jack',3))
MsgBox(0,'',_StringInsert($t,'Jack',-5))

[guinness]

Fixed by revision [11419] in version: 3.3.15.1