virsh.pod: Update find-storage-pool-sources[-as] man page

https://bugzilla.redhat.com/show_bug.cgi?id=921426

Add to the man page a more complete description of what exactly the
command expects on input and will return on output based on what is
currently supported.

Perhaps missing findPoolSources implementations are backends for
sheepdog and rbd.  Also missing any backend is zfs.

tools/virsh.pod

@@ -2954,16 +2954,49 @@ pools are similar to the ones used for domains.
 
 =item B<find-storage-pool-sources> I<type> [I<srcSpec>]
 
-Returns XML describing all storage pools of a given I<type> that could
-be found.  If I<srcSpec> is provided, it is a file that contains XML
-to further restrict the query for pools.
+Returns XML describing all possible available storage pool sources that
+could be used to create or define a storage pool of a given I<type>. If
+I<srcSpec> is provided, it is a file that contains XML to further restrict
+the query for pools.
+
+Not all storage pools support discovery in this manner. Furthermore, for
+those that do support discovery, only specific XML elements are required
+in order to return valid data, while other elements and even attributes
+of some elements are ignored since they are not necessary to find the pool
+based on the search criteria. The following lists the supported I<type>
+options and the expected minimal XML elements used to perform the search.
+
+For a "netfs" or "gluster" pool, the minimal expected XML required is the
+<host> element with a "name" attribute describing the IP address or hostname
+to be used to find the pool. The "port" attribute will be ignored as will
+any other provided XML elements in I<srcSpec>.
+
+For a "logical" pool, the contents of the I<srcSpec> file are ignored,
+although if provided the file must at least exist.
+
+For an "iscsi" pool, the minimal expect XML required is the <host> element
+with a "name" attribute describing the IP address or hostname to be used to
+find the pool (the iSCSI server address). Optionally, the "port" attribute
+may be provided, although it will default to 3260. Optionally, an <initiator>
+XML element with a "name" attribute may be provided to further restrict the
+iSCSI target search to a specific initiator for multi-iqn iSCSI storage pools.
 
 =item B<find-storage-pool-sources-as> I<type> [I<host>] [I<port>]
 [I<initiator>]
 
-Returns XML describing all storage pools of a given I<type> that could
-be found.  If I<host>, I<port>, or I<initiator> are provided, they control
-where the query is performed.
+Rather than providing I<srcSpec> XML file for B<find-storage-pool-sources>
+use this command option in order to have virsh generate the query XML file
+using the optional arguments. The command will return the same output
+XML as B<find-storage-pool-sources>.
+
+Use I<host> to describe a specific host to use for networked storage, such
+as netfs, gluster, and iscsi I<type> pools.
+
+Use I<port> to further restrict which networked port to utilize for the
+connection if required by the specific storage backend, such as iscsi.
+
+Use I<initiator> to further restrict the iscsi I<type> pool searches to
+specific target initiators.
 
 =item B<pool-autostart> I<pool-or-uuid> [I<--disable>]