Fix typos in documentation

Fix typos in documentation

In the porting guide, there are blue boxes that describe the argument
types and the return type of each function.  A small typo caused some of
these boxes not being properly rendered.

In the user guide, small typos were fixed that caused random text being
unintentionally rendered as bold.  Also, a slight rewording was done in
the section describing the DYN_DISABLE_AUTH flag.

Change-Id: I57303ca609436a82162fa9b42c06b0d5a63da6df
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>