virDomainDetachDeviceFlags: Clarify update semantics

https://bugzilla.redhat.com/show_bug.cgi?id=1621910

When users want to update a path to a CDROM they tend to
construct a very minimal XML and feed the API with it. This is
not a good practice as it breaks the assumptions the API is built
on. Most notably, leaving an element out should be treated as a
request for removal of the corresponding setting. Just like
leaving out <bandwidth/> clears out any QoS previously set.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

src/libvirt-domain.c

@@ -8318,6 +8318,14 @@ virDomainDetachDeviceFlags(virDomainPtr domain,
  * media, altering the graphics configuration such as password,
  * reconfiguring the NIC device backend connectivity, etc.
  *
+ * The supplied XML description of the device should contain all
+ * the information that is found in the corresponding domain XML.
+ * Leaving out any piece of information may be treated as a
+ * request for its removal, which may be denied. For instance,
+ * when users want to change CDROM media only for live XML, they
+ * must provide live disk XML as found in the corresponding live
+ * domain XML with only the disk path changed.
+ *
  * Returns 0 in case of success, -1 in case of failure.
  */
 int