doc: Add limitations and improvements for demo application

The main changes are as below:

* Move the description of running demo application manually from
  Reproduce section to Application section.
* Add a section to describe the limitations in rpmsg-demo and how
  to improve it.
* Update Known Issues and Limitations in changelog.
* Minor changes to keep the doc consistent overall.

Issue-Id: SCM-5651
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: Ia2c820f8e42f1251c6464e351f1583fd81fbb922