[Doc] Update LangRef for new Module Flag Behavior

Summary:
Add the documentation for the new module flag behavior. The new
ModFlagBehavior is added in r303590.

Reviewers: tejohnson

Reviewed By: tejohnson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36557

llvm-svn: 310926

llvm/docs/LangRef.rst

@@ -5378,6 +5378,10 @@ The following behaviors are supported:
            nodes. However, duplicate entries in the second list are dropped
            during the append operation.
 
+   * - 7
+     - **Max**
+           Takes the max of the two values, which are required to be integers.
+
 It is an error for a particular unique flag ID to have multiple behaviors,
 except in the case of **Require** (which adds restrictions on another metadata
 value) or **Override**.