Doxygen: Various improvements in libsigrok.h.

Doxygen: Various improvements in libsigrok.h.

 - Turn SR_OK/SR_ERR_* and SR_LOG_* into proper enums.

 - Use /**< Foo. */ for Doxygen comments that are on the same line as
   the (e.g.) enum entry they document. If the comment is not on the
   same line (but rather directly above the enum entry) a simple
   /** Foo. */ comment is sufficient.

 - Use /** */ instead of /* */ in some places, so that Doxygen actually uses
   the respective comments.

 - Various smaller cosmetic fixes or cleanups.