Update faq.md on underscore to mention `DISABLED_` prefix.
PiperOrigin-RevId:
334507963
## Why should test suite names and test names not contain underscore?
+Note: Googletest reserves underscore (`_`) for special purpose keywords, such as
+[the `DISABLED_` prefix](advanced.md#temporarily-disabling-tests), in addition
+to the following rationale.
+
Underscore (`_`) is special, as C++ reserves the following to be used by the
compiler and the standard library: