bridge.8: document fdb replace command

Despite commit 45a82e5 ("iproute vxlan add support for fdb replace
command"), the 'fdb replace' command was not mentioned in bridge.8.
Signed-off-by: Phil Sutter <phil@nwl.cc>

man/man8/bridge.8

@@ -50,7 +50,7 @@ bridge \- show / manipulate bridge addresses and devices
 .IR DEV " ]"
 
 .ti -8
-.BR "bridge fdb" " { " add " | " append " | " del " } "
+.BR "bridge fdb" " { " add " | " append " | " del " | " replace " } "
 .I LLADDR
 .B  dev
 .IR DEV " { "
@@ -407,6 +407,13 @@ This command removes an existing fdb entry.
 The arguments are the same as with
 .BR "bridge fdb add" ,
 
+.SS bridge fdb replace - replace a forwarding database entry
+If no matching entry is found, a new one will be created instead.
+
+.PP
+The arguments are the same as with
+.BR "bridge fdb add" ,
+
 .SS bridge fdb show - list forwarding entries.
 
 This command displays the current forwarding table.