doc: Document `update-source <IFNAME>` command better

There was a question in regards to how the update-source
choose the ip address for the source when using the `update-source`
command in BGP.  Upon looking at the code, I was a but surprised,
so I decided to document this behavior.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

diff --git a/doc/user/bgp.rst b/doc/user/bgp.rst
index 66096f226daa168a297d43deece4972c38fc7322..86d7c402edba098c670debf38b194fb906c8f624 100644 (file)
--- a/doc/user/bgp.rst
+++ b/doc/user/bgp.rst
@@ -1626,10 +1626,12 @@ Configuring Peers
 
 .. clicmd:: neighbor PEER update-source <IFNAME|ADDRESS>
 
-   Specify the IPv4 source address to use for the :abbr:`BGP` session to this
-   neighbour, may be specified as either an IPv4 address directly or as an
+   Specify the IPv4 or IPv6 source address to use for the :abbr:`BGP` session to this
+   neighbour, may be specified as either an IP address directly or as an
    interface name (in which case the *zebra* daemon MUST be running in order
-   for *bgpd* to be able to retrieve interface state).
+   for *bgpd* to be able to retrieve interface state).  When there are multiple
+   addresses on the choosen IFNAME then BGP will use the address that matches
+   the most number of bits in comparison to the destination peer address.
 
    .. code-block:: frr