Make use of HTML in outgoing emails optional

This will make it possible for gerrit-review.googlesource.com to turn
HTML off in the emails it sends until bugs in its EmailSender are
ironed out.

Bug: Issue 4589
Change-Id: Iadcfb5a38ac14044e0163a3f2db0c326ccda1f73
diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index f66f47e..8661d40 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -3327,6 +3327,14 @@
 +
 By default, true, allowing notifications to be sent.
 
+[[sendemail.html]]sendemail.html::
++
+If false, Gerrit will only send plain-text emails.
+If true, Gerrit will send multi-part emails with an HTML and
+plain text part.
++
+By default, true, allowing HTML in the emails Gerrit sends.
+
 [[sendemail.connectTimeout]]sendemail.connectTimeout::
 +
 The connection timeout of opening a socket connected to a