dpkg-query(1): Describe virtual fields

Add short description to each virtual field, and mention the version
they got introduced in dpkg.

debian/changelog

@@ -4,6 +4,9 @@ dpkg (1.17.12) UNRELEASED; urgency=low
   * Only build the compatibility selinux code if libselinux is available or
     the user requested it. Also fixes build failures on non-Linux systems.
     Closes: #757637
+  * Documentation:
+    - Add a short description to each virtual field in dpkg-query(1), and
+      mention the version they got introduced in dpkg.
 
   [ Updated programs translations ]
   * German (Sven Joachim).

man/dpkg-query.1

@@ -225,18 +225,40 @@ fields or fields stored in the binary package end up in it):
 The following are virtual fields, generated by \fBdpkg\-query\fP from
 values from other fields (note that these do not use valid names for
 fields in control files):
-
-.nf
-    \fBbinary:Package\fP
-    \fBbinary:Summary\fP
-    \fBdb:Status\-Abbrev\fP
-    \fBdb:Status\-Want\fP
-    \fBdb:Status\-Status\fP
-    \fBdb:Status\-Eflag\fP
-    \fBsource:Package\fP
-    \fBsource:Version\fP
-.fi
-
+.RS
+.TP
+.B binary:Package
+It contains the binary package name, in a multiarch conformant form,
+arch-qualified if needed (since dpkg 1.16.2).
+.TP
+.B binary:Summary
+It contains the package short description (since dpkg 1.16.2).
+.TP
+.B db:Status\-Abbrev
+It contains the abbreviated package status, such as \(lqii\(rq
+(since dpkg 1.16.2).
+.TP
+.B db:Status\-Want
+It contains the package wanted status, part of the Status field
+(since dpkg 1.17.11).
+.TP
+.B db:Status\-Status
+It contains the package status word, part of the Status field
+(since dpkg 1.17.11).
+.TP
+.B db:Status\-Eflag
+It contains the package status error flag, part of the Status field
+(since dpkg 1.17.11).
+.TP
+.B source:Package
+It contains the source package name for this binary package
+(since dpkg 1.16.2).
+.TP
+.B source:Version
+It contains the source package version for this binary package
+(since dpkg 1.16.2)
+.RE
+.IP
 The default format string is \(lq\fB${binary:Package}\et${Version}\en\fP\(rq.
 Actually, all other fields found in the status file (i.e. user defined
 fields) can be requested, too. They will be printed as-is, though, no