man: Packages are automatically forgotten only if they contain no user data

If a package in the database contains a selection for example, then it
will not be forgotten.

Closes: #813179

debian/changelog

@@ -48,6 +48,8 @@ dpkg (1.18.5) UNRELEASED; urgency=medium
     - Document and improve C/C++ programs exit codes in man pages.
     - Clarify dpkg --path-exclude/--path-include pathname filter behavior.
       Closes: #811267
+    - Clarify that packages are only automatically forgotten by dpkg if they
+      contain no user data, such as package selections. Closes: #813179
 
   [ Updated programs translations ]
   * Portuguese (Miguel Figueiredo).

man/dpkg.1

@@ -278,7 +278,8 @@ must refer to a directory instead.
 .TP
 .B \-\-forget\-old\-unavail
 Now \fBobsolete\fP and a no-op as \fBdpkg\fP will automatically forget
-uninstalled unavailable packages (since dpkg 1.15.4).
+uninstalled unavailable packages (since dpkg 1.15.4), but only those that
+do not contain user information such as package selections.
 .TP
 .B \-\-clear\-avail
 Erase the existing information about what packages are available.