Update documentation.

src/vs/editor/contrib/snippet/snippet.md

@@ -53,6 +53,26 @@ ${TM_FILENAME/(.*)\..+$/$1/}
   |-> resolves to the filename
 ```
 
+Placeholder-Transform
+--
+
+Like a Variable-Transform, a transformation of a placeholder allows changing the inserted text for the placeholder when moving to the next tab stop.
+The inserted text is matched with the regular expression and the match or matches - depending on the options - are replaced with the specified replacement format text.
+Every occurrence of a placeholder can define its own transformation independently using the value of the first placeholder.
+The format for Placeholder-Transforms is the same as for Variable-Transforms.
+
+The following sample removes an underscore at the beginning of the text. `_transform` becomes `transform`.
+
+```
+${1/^_(.*)/$1/}
+  |   |    |  |-> No options
+  |   |    |
+  |   |    |-> Replace it with the first capture group
+  |   |
+  |   |-> Regular expression to capture everything after the underscore
+  |
+  |-> Placeholder Index
+```
 
 Grammar
 --
@@ -61,12 +81,17 @@ Below is the EBNF for snippets. With `\` (backslash) you can escape `$`, `}` and
 
 ```
 any         ::= tabstop | placeholder | choice | variable | text
-tabstop     ::= '$' int | '${' int '}'
+tabstop     ::= '$' int
+                | '${' int '}'
+                | '${' int  transform '}'
 placeholder ::= '${' int ':' any '}'
+                | '${' int ':' any transform '}'
 choice      ::= '${' int '|' text (',' text)* '|}'
+                | '${' int '|' text (',' text)* '|' transform '}'
 variable    ::= '$' var | '${' var }'
                 | '${' var ':' any '}'
-                | '${' var '/' regex '/' (format | text)+ '/' options '}'
+                | '${' var transform '}'
+transform   ::= '/' regex '/' (format | text)+ '/' options
 format      ::= '$' int | '${' int '}'
                 | '${' int ':' '/upcase' | '/downcase' | '/capitalize' '}'
                 | '${' int ':+' if '}'
@@ -78,3 +103,5 @@ var         ::= [_a-zA-Z] [_a-zA-Z0-9]*
 int         ::= [0-9]+
 text        ::= .*
 ```
+
+Transformations for placeholders and choices are an extension to the TextMate snippet grammar and only support by Visual Studio Code.