clarify difference between failover and round-robin mail transports

Author: pableu

mailer.rst

@@ -210,7 +210,7 @@ A failover transport is configured with two or more transports and the
 
     MAILER_DSN="failover(postmark+api://ID@default sendgrid+smtp://KEY@default)"
 
-The mailer will start using the first transport and if it fails, it
+The failover-transport starts using the first transport and if it fails, it
 will retry the same delivery with the next transports until one of them succeeds
 (or until all of them fail).
 
@@ -227,8 +227,12 @@ A round-robin transport is configured with two or more transports and the
 
     MAILER_DSN="roundrobin(postmark+api://ID@default sendgrid+smtp://KEY@default)"
 
-The round-robin transport is similar to the failover transport (see above), but
-it starts with a *randomly* selected transport instead of the first in the list.
+The round-robin transport starts with a *randomly* selected transport and
+then switches to the next available transport for each subsequent email.
+
+As with the failover transport, round-robin retries deliveries until
+a transport succeeds (or all failed). In contrast to the failover transport,
+it  *balances* the load between all its transports.
 
 .. versionadded:: 5.1