doc: Improve documentation

Change paragraphs to more guiding directives. Make consistent use of
action verbs throughout the reproduce page.

List newly found issues with the demo.

Fix grammar.

Reinforce the importance of X11 forwarding in the relevant sections of
the reproduce page.

Fix mistakes in the commands.

Add more information on the expected output and expected warnings for
the commands the user has to run.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>