#3121: add doc for imap/smtp timeout

Documentation/SOGoInstallationGuide.asciidoc

@@ -2394,6 +2394,26 @@ Defaults to `/tmp`.
 
 The default value is `YES`.
 
+|S |NGImap4ConnectionTimeout
+|Timeout, in seconds, of the IMAP connections (primary and auxiliary
+accounts). It applies to the connection itself, to the TLS handshake and
+to every read and write on the connection, so a server that stops answering
+does not hold a SOGo worker indefinitely. Set it to `0` to disable the
+timeout. Increase it if some IMAP commands (for example searches on very
+large mailboxes) can take longer than that without any output from the
+server.
+
+The default value is `60`.
+
+|S |NGSmtpConnectionTimeout
+|Timeout, in seconds, of the SMTP connections (primary and auxiliary
+accounts). It applies to the connection itself, to the TLS handshake and
+to every read and write on the connection. Set it to `0` to disable the
+timeout. Increase it if your SMTP server (for example with content filters)
+can take longer than that to answer after the message data has been sent.
+
+The default value is `60`.
+
 |S |NGImap4AuthMechanism
 |Trigger the use of the IMAP `AUTHENTICATE` command with the specified
 SASL mechanism. Using `AUTHENTICATE` instead of `LOGIN` is also necessary
@@ -2679,6 +2699,13 @@ Defaults to `manually` when unset.
 set to `YES`, users can add other IMAP accounts that will be visible
 from the SOGo Webmail interface.
 
+As users choose the IMAP and SMTP servers of these accounts, a server that
+accepts connections but never answers holds a SOGo worker until
+_NGImap4ConnectionTimeout_ (or _NGSmtpConnectionTimeout_) is reached. When
+enabling this parameter, consider also limiting the requests per user with
+_SOGoMaximumRequestCount_ and, in front of SOGo, the requests per IP address
+in the reverse proxy.
+
 Defaults to `NO` when unset.
 
 |D |SOGoMailAuxiliaryUserAccountsReadOnly