adding doxygen documentations to all libcli.h header functions

[mpzanoosi]

Hi I used your library somewhere and I found a Doxygen documentation can help a lot! So I've added Doxygen to all libcli.h functions! I've also added Doxygen configuration file conf.doxygen which can be used with command doxygen conf.doxygen then the documentation can be found in doc/ folder