[tr064] improve documentation and add textual config example (#9648)

[tr064] improve documentation and add textual config example (#9648)

* Expanded descriptions and note on textual config

Some paragraphs are expanded and some more info on the fritzbox subdevices are given.  Channels are grouped by subdevice.  Textual description with .items files is briefly described at the end but a clear warning on possible drawbacks is given and it is also clearly stated that it is discouraged.

Signed-of-by: Lionello Marrelli <marrelli_lio@yahoo.it>