Update Toast documentation

to explicitly callout the 2-line limit
Note: more information can be found in
https://developer.android.com/guide/topics/ui/notifiers/toasts
which is also linked in the documentation but
could be missed.

Test: manual
Fixes: 207767563
Change-Id: Icc7fbf6fddf101bd2422bbeab04e0da8084c6698