Adds comment clarifying the use of default-constructed matchers.

include/gmock/gmock-matchers.h

@@ -250,8 +250,9 @@ class MatcherBase {
 template <typename T>
 class Matcher : public internal::MatcherBase<T> {
  public:
-  // Constructs a null matcher.  Needed for storing Matcher objects in
-  // STL containers.
+  // Constructs a null matcher.  Needed for storing Matcher objects in STL
+  // containers.  A default-constructed matcher is not yet initialized.  You
+  // cannot use it until a valid value has been assigned to it.
   Matcher() {}
 
   // Constructs a matcher from its implementation.