odk: document what doxygen means by "Protected Member" in IDL

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

odk/docs/idl/main.dox

@@ -9,3 +9,12 @@
 /// \mainpage
 ///
 /// This is a reference documentation for the UNO IDL API.
+///
+/// \remark
+/// UNO IDL has no concept of "public" or "protected".
+/// "Protected Member" or "Protected Attribute" in the context
+/// of this IDL documentation actually means
+/// "members inherited from optional service or interface",
+/// i.e., not every implementation supports these members
+/// or attributes.
+