improve documentation for mailer failover and round-robin transports

Author: pableu

mailer.rst

@@ -210,9 +210,9 @@ 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. If the sending fails, the
-mailer won't retry it with the other transports, but it will switch to the next
-transport automatically for the following deliveries.
+The mailer will start 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).
 
 Load Balancing
 ~~~~~~~~~~~~~~
@@ -227,9 +227,8 @@ 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 mailer will start using a randomly selected 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).
+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.
 
 .. versionadded:: 5.1