Add ssh_connection retries to ansible.cfg example (#38393)

I add the `retries` option under [ssh_connection] as it was missing, and
some brief comments on the backoff logic.

examples/ansible.cfg

@@ -406,6 +406,11 @@
 # requires a tty by default. 
 #use_tty = True
 
+# Number of times to retry an SSH connection to a host, in case of UNREACHABLE.
+# For each retry attempt, there is an exponential backoff,
+# so after the first attempt there is 1s wait, then 2s, 4s etc. up to 30s (max).
+#retries = 3
+
 [persistent_connection]
 
 # Configures the persistent connection timeout value in seconds.  This value is