docs(qemu): delineate flash based boot method

Make the language around the explanation for booting via secure flash
clearer. Provide details into the intent of the options given to QEMU.

Change-Id: Ia573b900aaa2346cad4f82191110b978f9bd5481
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
1 file changed