docstring: variable: Move docstring from source to header file

docstring: variable: Move docstring from source to header file

virConnectAuthPtrDefault is the only variable that we export and
with the documentation being at the C file, it makes harder to find
it.

With this patch, the documentation of this variable can be found by
those who look at the header file for guidance plus, it will also be
included in the XML API, as the followup patch will fix apibuild.py
to store what is documented in the header file for this variable.

Note that the apibuild, does not do the same with .c files. It'd
require adding it for (parsing) recursion, similar to what is done
for functions.  Considering the pros/cons, seems more reasonable to
move the docstring to the header file instead.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>