[FR]: add docs for GTEST_FLAG_SET and GTEST_FLAG_GET

[hexagonrecursion]

Does the feature exist in the most recent commit?

No

Why do we need this feature?

Without documentation it is not obvious whether this macros are a part of the public API or not.

GTEST_FLAG_SET is kinda mentioned in passing:

googletest/docs/advanced.md

Line 588 in 35d0c36

GTEST_FLAG_SET(death_test_style, "threadsafe");

GTEST_FLAG_GET is not documented at all

Describe the proposal.

Add two new entries to https://google.github.io/googletest/reference/testing.html - one to document GTEST_FLAG_SET and one to document GTEST_FLAG_GET.

Is the feature specific to an operating system, compiler, or build system version?

No