Document cache directory usage

[zero323]

When tests are executed with typecheck_in_new_subprocess and disable_cache is False, --cache-dir is passed directly to mypy call as a path relative to root directory.

This means that the following are ignored

• MYPY_CACHE_DIRin execution environment orenv` block of the test case.
• cache_dir in mypy configuration.

are ignored.

This behavior is confusing, so it might be a good idea to document it.

In a long run, YamlTestItem should check if any of these are provided, and omit --cache-dir in such cases (this would be useful for example with remote cache).