Enhance input documentation on embedded

Especially for keyboard. The main inspiration here is to document
the 'grab' parameter and make the Input section more readable.
In addition some notes about the other parameters and custom keymaps
are added too.

Change-Id: I0ded5b673c62d51b34e75d48246f0b58d805100a
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>