deb-control(5): Clarify Multi-Arch: no field omission

Reported-by: Helge Kreutzmann <debian@helgefjell.de>

debian/changelog

@@ -4,6 +4,7 @@ dpkg (1.17.7) UNRELEASED; urgency=low
   * Documentation:
     - Mention in deb-symbols(5), that the “main-dependency-template” is
       always used. Closes: #737731
+    - Clarify in deb-control(5) «Multi-Arch: no» field omission.
 
   [ Updated dpkg translations ]
   * German (Sven Joachim).

man/deb-control.5

@@ -129,7 +129,7 @@ value \fBallowed\fP allows reverse-dependencies to indicate in their
 Depends field that they accept a package from a foreign architecture, but
 has no effect otherwise.
 The value \fBno\fP is the default when the field is omitted, in which case
-the field is generally not needed.
+adding the field with an explicit \fBno\fP value is generally not needed.
 .TP
 .BI Source: " source-name"
 The name of the source package that this binary package came from, if