Add type manager synchronization comment.

This Cl adds a comment about the type manager being a singleton and
synchronization requirements.

Change-Id: I91618516b26c18aa6b47e7e564b4b3a8eecc1009
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/17740
Reviewed-by: David Neto <dneto@google.com>

src/type_manager.h

@@ -24,6 +24,9 @@
 namespace tint {
 
 /// The type manager holds all the pointers to the known types.
+///
+/// Note, the type manager is a singleton. Any synchronization for the manager
+/// must be done by the caller.
 class TypeManager {
  public:
   /// @returns a pointer to the type manager