Update docs: assertions are thread-safe on desktop Windows

[cindykrafft]

Fixes #4769

The primer's "Known Limitations" section says it is unsafe to use assertions from two threads concurrently on systems without pthreads "(e.g. Windows)". docs/advanced.md has two similar notes ("Assertions from multiple threads are currently not supported on Windows."), one after the {ASSERT|EXPECT}_NO_FATAL_FAILURE example and one after the EXPECT_{NON}FATAL_FAILURE_ON_ALL_THREADS macros.

These statements are outdated:

• gtest-port.h defines GTEST_IS_THREADSAFE for desktop Windows. The condition includes defined(GTEST_OS_WINDOWS) && !defined(GTEST_OS_WINDOWS_PHONE) && !defined(GTEST_OS_WINDOWS_RT).
• gtest-port.cc provides Windows implementations of Mutex, ThreadLocal and ThreadWithParam.
• The tests that exercise assertions from multiple threads are guarded by GTEST_IS_THREADSAFE, so they are also built on desktop Windows. Examples are ExpectFailureWithThreadsTest and the concurrent SCOPED_TRACE test in googletest-output-test_.cc, and the threading tests in gtest_stress_test.cc and googletest-port-test.cc.

This PR changes documentation only:

• The primer now says the implementation is thread-safe where pthreads is available and on desktop Windows, and unsafe on other systems.
• The two Windows-specific notes are removed from advanced.md.

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