docs: Enhance documentation of log_filters

Also make sure documentation in libvirtd.conf matches the one from
logging.html.

daemon/libvirtd.conf

@@ -291,10 +291,13 @@
 # Logging filters:
 # A filter allows to select a different logging level for a given category
 # of logs
-# The format for a filter is:
+# The format for a filter is one of:
 #    x:name
-#      where name is a match string e.g. remote or qemu
-# the x prefix is the minimal level where matching messages should be logged
+#    x:+name
+#      where name is a string which is matched against source file name,
+#      e.g., "remote", "qemu", or "util/json", the optional "+" prefix
+#      tells libvirt to log stack trace for each message matching name,
+#      and x is the minimal level where matching messages should be logged:
 #    1: DEBUG
 #    2: INFO
 #    3: WARNING

docs/logging.html.in

@@ -118,9 +118,12 @@
     <pre>
   x:name  (log message only)
   x:+name (log message + stack trace)</pre>
-    <p>where <code>name</code> is a match string e.g. <code>remote</code> or
-    <code>qemu</code> and the x is the minimal level where matching messages
-    should be logged:</p>
+    <p>where <code>name</code> is a string which is matched against source
+    file name, e.g., <code>remote</code>, <code>qemu</code>, or
+    <code>util/json</code>, the optional <code>+</code> prefix tells libvirt
+    to log stack trace for each message matching <code>name</code>, and
+    <code>x</code> is the minimal level where matching messages should
+    be logged:</p>
     <ul>
       <li>1: DEBUG</li>
       <li>2: INFO</li>