Document the ld, st, while statements.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@16588 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

doc/ffmpeg-doc.texi

@@ -830,6 +830,9 @@ The following binary operators are available: @code{+}, @code{-},
 The following unary operators are available: @code{+}, @code{-},
 @code{(...)}.
 
+The following statements are available: @code{ld}, @code{st},
+@code{while}.
+
 The following functions are available:
 @table @var
 @item sinh(x)