Add a documentation to the 'a' flag

doc/UltiSnips.txt

@@ -1152,6 +1152,11 @@ The options can be any combination of >
    i    - case insensitive
           By default, regular expression matching is case sensitive. With this
           option, matching is done without regard to case.
+   a    - ascii conversion
+          By default, transformation is made on the raw utf-8 string. With
+          this option, matching is done with a string converted into ascii.
+          One use of this option is to replace accentued chars by a non
+          accentued version. 
 
 The syntax of regular expressions is beyond the scope of this document. Python
 regular expressions are used internally, so the python 're' module can be used
@@ -1211,6 +1216,18 @@ this is a title
 This Is A Title
 
 
+Demo: ASCII transformation
+------------------- SNIP -------------------
+snippet ascii "Replace non ascii chars"
+${1: an accentued text}
+${1/.*/$0/a}
+endsnippet
+------------------- SNAP -------------------
+ascii<tab>à la pêche aux moules
+à la pêche aux moules
+a la peche aux moules
+
+
 Demo: Regular expression grouping
       This is a clever c-like printf snippet, the second tabstop is only shown
       when there is a format (%) character in the first tabstop.