doc: extend description of 'remote.via'

doc/configuration.rst

@@ -172,6 +172,7 @@ server. If the zone file doesn't exist it will be bootstrapped over AXFR::
     remote:
       - id: master
         address: 192.168.1.1@53
+        # via: 10.0.0.1            # Specify local source address if needed
 
     acl:
       - id: notify_from_master

doc/man/knot.conf.5in

@@ -1004,8 +1004,10 @@ no more addresses will be tried for this remote.
 .SS via
 .sp
 An ordered list of source IP addresses. The first address with the same family
-as the destination address is used. Optional source port (default is random)
-can be appended to the address using \fB@\fP separator.
+as the destination address is used as a source address for communication with
+the remote. This option can help if the server listens on more addresses.
+Optional source port (default is random) can be appended
+to the address using \fB@\fP separator.
 .sp
 \fIDefault:\fP not set
 .SS key

doc/reference.rst

@@ -1056,8 +1056,10 @@ via
 ---
 
 An ordered list of source IP addresses. The first address with the same family
-as the destination address is used. Optional source port (default is random)
-can be appended to the address using ``@`` separator.
+as the destination address is used as a source address for communication with
+the remote. This option can help if the server listens on more addresses.
+Optional source port (default is random) can be appended
+to the address using ``@`` separator.
 
 *Default:* not set