* Clarify document generation steps

Signed-off-by: Dwayne Robinson <dwayner@microsoft.com>

* Add note about stat_coverage.py, which is undocumented but also essential for the checkin to succeed

Signed-off-by: Dwayne Robinson <dwayner@microsoft.com>

Co-authored-by: Chun-Wei Chen <jacky82226@gmail.com>