TableGen/LangRef: link bang operators into the productionlist

Now BangOperator should be nicely hyperlinked.

Pointed out by Joel Jones.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171942 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/TableGen/LangRef.rst b/docs/TableGen/LangRef.rst
index 506d207..92fdb4a 100644
--- a/docs/TableGen/LangRef.rst
+++ b/docs/TableGen/LangRef.rst
@@ -86,11 +86,13 @@
    int   let       list    multiclass   string
 
 TableGen also has "bang operators" which have a
-wide variety of meanings::
+wide variety of meanings:
 
-   !eq     !if      !head    !tail      !con
-   !shl    !sra     !srl
-   !cast   !empty   !subst   !foreach   !strconcat
+.. productionlist::
+   BangOperator: one of
+               :!eq     !if      !head    !tail      !con
+               :!shl    !sra     !srl
+               :!cast   !empty   !subst   !foreach   !strconcat
 
 Syntax
 ======