doc: update description about user/group in qemu.conf

As a side effect of changes in the functions virGetUserID and
virGetGroupID, the user and group configurations for DAC in qemu.conf
are now able to accept both names and IDs, supporting a leading plus
sign to ensure that a numeric value will not be interpreted as a name.

This patch updates the comments in qemu.conf, including a description of
this new behavior.

src/qemu/qemu.conf

@@ -180,10 +180,25 @@
 # guests will be blocked. Defaults to 0.
 #security_require_confined = 1
 
-# The user ID for QEMU processes run by the system instance.
+# The user for QEMU processes run by the system instance. It can be
+# specified as a user name or as a user id. The qemu driver will try to
+# parse this value first as a name and then, if the name doesn't exist,
+# as a user id.
+#
+# Since a sequence of digits is a valid user name, a leading plus sign
+# can be used to ensure that a user id will not be interpreted as a user
+# name.
+#
+# Some examples of valid values are:
+#
+#       user = "qemu"   # A user named "qemu"
+#       user = "+0"     # Super user (uid=0)
+#       user = "100"    # A user named "100" or a user with uid=100
+#
 #user = "root"
 
-# The group ID for QEMU processes run by the system instance.
+# The group for QEMU processes run by the system instance. It can be
+# specified in a similar way to user.
 #group = "root"
 
 # Whether libvirt should dynamically change file ownership