Missing documentation for InitGoogleMock

**Describe the bug**

--gmock_verbose=LEVEL option is mentioned in documentation but to work it either requirres using gmock_main.a or calling ::testing::InitGoogleMock in main.

**Steps to reproduce the bug**
Compile any test suite using gtest_main.a or using testing::InitGoogleTest in main to initialize gtest.

**What version of GoogleTest are you using?**
1.10
**What operating system and version are you using**
Linux

**What compiler and version are you using?**
gcc 9.3.0

**What build system are you using?**
cmake 3.17

**Additional context**
None

Start by locating the documentation that mentions the --gmock_verbose=LEVEL option and review how gtest_main.a and InitGoogleTest are described. Update the documentation to explain when InitGoogleMock or gmock_main.a is required, then verify that the documented setup matches the reported GoogleTest 1.10 behavior.

Written by the indexing model from the issue text.