Update documentation for "Lightweight mode"
Update documentation for "Lightweight mode"

This was misleading some users into thinking that lightweight mode
performed differently on-device than normal mode. This update
changes the name of that mode to "simple mode" and clarifies that
there is no behavior change compared to "normal mode".

Bug: n/a
Change-Id: Iff2dbc038a90b8d57278d73218ec3d48650483b0
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto