docs: update to properly reflect meaning of fields in log filter

docs: update to properly reflect meaning of fields in log filter

The documentation (and comment in libvirtd.conf) says that the text in
a log filter is compared to the "source file name", and gives the
example of "util/json", but this is not correct (at least not since
commit 2835c1e, possibly earlier). It is instead compared to the
string given in the VIR_LOG_INIT() macro invocation at the top of each
source file, which is always "similar to but not the same as" the
source file name (in the example above, the proper name is
"util.json", while the file name is "util/virjson.c"). This patch
corrects the misstatement in both the documentation and in
libvirtd.conf.