Interfaces and services that are marked as "optional" in IDL are
deliberately tagged as "Protected" by doxygen, while everything else is
"Public", so that in the inheritance diagrams dashed lines are drawn
for "optional" bases and solid lines for non-optional bases.

Unfortunately this also causes doxygen to produce the text "Protected
Members", which appears hard to change, so just document it for now.

Change-Id: Ice878981bac5bbb7a8a33a10f2b5f68c394e4340