Improve am --help wording

-W flag is explained as "wait for launch to complete", which is vague.
Adding "(initial display)" is searchable and actually brings up
https://developer.android.com/topic/performance/vitals/launch-time
which explains very well when is initial display occurding and therefore
when "am -W" returns.

Test: NA
Bug: NA
Change-Id: I87f51206382addd850b0efba098f733da79b4902