Suggest developers use -eng builds instead when they lunch -userdebug

This is because -eng provides faster local builds.

(also moving hints to where they are more likely to be seen)

Test: lunch cf_x86_64_phone-userdebug
Fixes: 307753064
Change-Id: I52cf9e9c399dbb3a52c7952f7204a9e567d15c22