DOC: Make how "option redispatch" works more explicit

People are often misled and think that this option can redirect
connections to backup servers.

This patch makes the documentation more specific about how the option
handles backup servers.

doc/configuration.txt

@@ -7616,6 +7616,26 @@ no option redispatch
   Specifying "option redispatch" will allow the proxy to break cookie or
   consistent hash based persistence and redistribute them to a working server.
 
+  Active servers are selected from a subset of the list of available
+  servers. Active servers that are not down or in maintenance (i.e., whose
+  health is not checked or that have been checked as "up"), are selected in the
+  following order:
+
+    1. Any active, non-backup server, if any, or,
+
+    2. If the "allbackups" option is not set, the first backup server in the
+       list, or
+
+    3. If the "allbackups" option is set, any backup server.
+
+  When a retry occurs, HAProxy tries to select another server than the last
+  one. The new server is selected from the current list of servers.
+
+  Sometimes, if the list is updated between retries (e.g., if numerous retries
+  occur and last longer than the time needed to check that a server is down,
+  remove it from the list and fall back on the list of backup servers),
+  connections may be redirected to a backup server, though.
+
   It also allows to retry connections to another server in case of multiple
   connection failures. Of course, it requires having "retries" set to a nonzero
   value.