man: Add a rationale for the purpose of Testsuite-Triggers in dsc(5)

Prompted-by: Mattia Rizzolo <mattia@debian.org>

debian/changelog

@@ -3,6 +3,8 @@ dpkg (1.18.18) UNRELEASED; urgency=medium
   [ Guillem Jover ]
   * Documentation:
     - Document DPKG_DATADIR in an ENVIRONMENT section in dpkg-architecture(1).
+    - Add a rationale for the purpose of Testsuite-Triggers in dsc(5).
+      Prompted by Mattia Rizzolo <mattia@debian.org>.
   * Packaging:
     - Revert the perl to perl:any dependency change in libdpkg-perl, as that
       breaks debootstrap in stable (jessie), testing (stretch) and unstable,

man/dsc.man

@@ -149,6 +149,9 @@ This field declares the comma-separated union of all test dependencies
 restrictions removed, and OR dependencies flattened, except for binaries
 generated by this source package and meta-dependencies such as \fB@\fP or
 \fB@builddeps@\fP.
+
+Rationale: this field is needed because otherwise to be able to get the
+test dependencies, each source package would need to be unpacked.
 .TP
 .BI Build\-Depends: " package-list"
 .TQ