docs: document MOCK_METHOD argument limit

Fixes #4608

MOCK_METHOD accepts at most 31 arguments (doubled from 15 by
dc3c9eda). Document this limit in gmock_cook_book.md and the
MOCK_METHOD reference.

docs/gmock_cook_book.md

@@ -15,6 +15,8 @@ brevity, but you should do it in your own code.
 Mock classes are defined as normal classes, using the `MOCK_METHOD` macro to
 generate mocked methods. The macro gets 3 or 4 parameters:
 
+Note that a mocked method can have at most 31 arguments.
+
 ```cpp
 class MyMock {
  public:

docs/reference/mocking.md

@@ -16,6 +16,8 @@ GoogleTest defines the following macros for working with mocks.
 Defines a mock method *`method_name`* with arguments `(`*`args...`*`)` and
 return type *`return_type`* within a mock class.
 
+The number of arguments is limited to 31.
+
 The parameters of `MOCK_METHOD` mirror the method declaration. The optional
 fourth parameter *`specs...`* is a comma-separated list of qualifiers. The
 following qualifiers are accepted: