dpkg: Document packages w/o Architecture field need --force-architecture

The commit 58e7276b69be105735eccb3c18c2b28d3a2ec2e5 didn't explicitly
document this behavioural change, do so now.

debian/changelog

@@ -28,6 +28,8 @@ dpkg (1.16.0) UNRELEASED; urgency=low
   * Always warn when parsing any package control data which does not have
     an Architecture field except for status and status log files when
     packages are not-installed or half-installed.
+  * By default reject installing packages w/o an Architecture field. They
+    now need --force-architecture, dpkg will still warn about them though.
 
   [ Raphaël Hertzog ]
   * Fail properly when debian/source/format is empty. Closes: #600854

man/dpkg.1

@@ -463,7 +463,7 @@ any software not doing syncs before atomic renames.
 losing data, use with care.\fP
 
 \fBarchitecture\fP:
-Process even packages with the wrong architecture.
+Process even packages with wrong or no architecture.
 
 \fBbad\-path\fP:
 \fBPATH\fP is missing important programs, so problems are likely.

src/main.c

@@ -436,7 +436,7 @@ static void setforce(const struct cmdinfo *cip, const char *value) {
 "  confask [!]            Offer to replace config files with no new versions\n"
 "  breaks [!]             Install even if it would break another package\n"
 "  conflicts [!]          Allow installation of conflicting packages\n"
-"  architecture [!]       Process even packages with wrong architecture\n"
+"  architecture [!]       Process even packages with wrong or no architecture\n"
 "  overwrite-dir [!]      Overwrite one package's directory with another's file\n"
 "  unsafe-io [!]          Do not perform safe I/O operations when unpacking\n"
 "  remove-reinstreq [!]   Remove packages which require installation\n"