Simplify "adb sync" documentation.

Remove -p (which only applied to "adb sync"), explicitly mention
$ANDROID_PRODUCT_OUT in the "adb sync" help text, and add "adb sync all"
as a synonym for "adb sync" for ease of documentation.

Bug: http://b/33761074
Test: "adb sync" still works, "adb --help" looks good
Change-Id: Ie5b2459a7e5e4140298f78815be25fa7c7bb47d0