usb: raw-gadget: update documentation and Kconfig

Update Raw Gadget documentation and Kconfig. Make the description more
precise and clear, fix typos and grammar mistakes, and do other cleanups.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Link: https://lore.kernel.org/r/f4c650c94ae2b910e38819d51109cd5f0b251a2a.1611429174.git.andreyknvl@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>