Wifi usability: Add expected range for Wi-Fi usability score

In addition, fix a couple of typos in documentation.

Bug: 126391553
Bug: 126699865
Bug: 113262380
Bug: 128675382

Test: frameworks/base/wifi/tests/runtests.sh

Change-Id: I4c8d73f66f7d2d424bef3699dfc889e342621b02
Signed-off-by: Mingguang Xu <mingguangxu@google.com>