UPSTREAM: usb: raw-gadget: documentation updates

Mention the issue with fixed UDC addresses.

Links external examples and test suite.

Add more implmenetation details and potential improvements.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Bug: 147413187
(cherry picked from commit 61d2658db48af0c563d126cccc5c1105137b1ec9)
Change-Id: I99ec38c7fc69aba4e9019a320b740843dd262a48
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>