docs: Normalize Doxygen paragraph indentation

Doxygen continutation line intendation across the code-base is currently
inconsistent in spite of the code-style documentation specifying a
two-tab indent on continuation lines.

Change-Id: I0944d749042972797bde1997f02afd98d4028c1a
Signed-off-by: Chris Kay <chris.kay@arm.com>