Update doxygen config file

Update doxygen config file

Update doxygen config file to 1.6.1 + add comments + change following
settings:

OUTPUT_DIRECTORY = doc/generated
ALWAYS_DETAILED_SEC = YES - we want all function in detailed list
JAVADOC_AUTOBRIEF = YES - IMHO the least painfull way to extract brief
desxcription, see doxygen docs for the other ways. But still painful enough :(
OPTIMIZE_OUTPUT_FOR_C  = YES - we're C project
IDL_PROPERTY_SUPPORT   = NO - we're C project
WARN_NO_PARAMDOC = YES - document all function parameters please!
INPUT = include - generate public docs only
GENERATE_LATEX = NO - no LaTeX for now, maybe later
*_GRAPH = NO - no call/include graphs in public docs

Signed-off-by: Jan Safranek <jsafrane@redhat.com>