minor symfony#12301 [Mailer] Better explain the load-balancing options (javiereguiluz)

This PR was squashed before being merged into the 4.3 branch (closes symfony#12301).

Discussion
----------

[Mailer] Better explain the load-balancing options

Commits
-------

bc668cc [Mailer] Better explain the load-balancing options

Author: javiereguiluz

components/mailer.rst

@@ -100,24 +100,37 @@ binary. For third-party providers, refers to the following table:
  Sendgrid             smtp://apikey:KEY@sendgrid         n/a                                api://KEY@sendgrid
 ==================== ================================== ================================== ================================
 
-Failover Transport
-------------------
+Load Balancing
+--------------
 
-You can create failover transport with the help of `||` operator::
+Symfony's mailer supports `load balancing`_ so you can distribute the mailing
+workload across multiple transports. There are two main techniques to balance
+the load: failover and round-robin.
+
+Failover Load Balancing
+~~~~~~~~~~~~~~~~~~~~~~~
+
+A failover transport is configured with two or more transports joined by the
+``||`` operator::
 
     $dsn = 'api://id@postmark || smtp://key@sendgrid';
 
-So if the first transport fails, the mailer will attempt to send through the
-second transport.
+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.
 
-Round Robin
------------
+Round-Robin Load Balancing
+~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-If you want to send emails by using multiple transports in a round-robin fashion,
-you can use the ``&&`` operator between the transports::
+A round-robin transport is configured with two or more transports joined by the
+``&&`` operator::
 
     $dsn = 'api://id@postmark && smtp://key@sendgrid'
 
+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).
+
 Sending emails asynchronously
 -----------------------------
 
@@ -167,3 +180,5 @@ Learn More
 
 To learn more about how to use the mailer component, refer to the
 :doc:`Symfony Framework Mailer documentation </mailer>`.
+
+.. _`load balancing`: https://en.wikipedia.org/wiki/Load_balancing_(computing)