Improve doxygen

* Use Doxygen groups for each group of header attributes, optional block
  values, flags, or any other logic grouping of definitions. This groups
  these definitions in the output as well as allows convenient anchors
  by which to refer to these groups from elsewhere.
* Reduce the mentions of "TR-31". The TR-31 specification is deprecated
  and therefore the name "TR-31" should only refer to the library itself
  or to types provided by the library. The actual key block concepts and
  functionality now involve multiple other specifications while only the
  library itself is still called "TR-31".
* Add tr31_strings.h to Doxygen generator

CMakeLists.txt

@@ -176,6 +176,7 @@ if(Doxygen_FOUND)
 	if(BUILD_DOCS)
 		doxygen_add_docs(docs
 			tr31.h
+			tr31_strings.h
 			ALL # build by default
 			WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/src"
 		)
@@ -189,6 +190,7 @@ if(Doxygen_FOUND)
 	else()
 		doxygen_add_docs(docs
 			tr31.h
+			tr31_strings.h
 			WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/src"
 		)
 	endif()