doc: zbus: Fix typos and apply general docs improvement

Add details for several unclear aspects of zbus. The explanation of VDED
was vague, and some developers needed clarification. So this commit adds
images and an illustration scenario to explain that in detail. Typos
corrected. More code was added in the claim/finish descriptions.

Signed-off-by: Rodrigo Peixoto <rodrigopex@gmail.com>