doc: exempt projects with manual createrepo from pruning old results

Fix #3104

We skipping such projects with the following explanation.

    Exploring projectdir 'kde6' with path: /var/lib/copr/public_html/results/solopasha/kde6
    Going to prune solopasha/kde6
    projectname = kde6
    Skipped solopasha/kde6 since auto createrepo option is disabled

So I am just documenting the status quo.

doc/user_documentation.rst

@@ -563,6 +563,7 @@ only in Copr and ignored by Koji. It is easy to do::
   # This happens only in Copr
   %endif
 
+.. _creating_repositories_manually:
 
 Creating repositories manually
 ------------------------------
@@ -767,6 +768,9 @@ even though that build might not be the newest one.  Also, if there are
 two builds of the same package version, it is undefined which one is going
 to be kept.
 
+Projects that opted-in for :ref:`creating_repositories_manually`, are
+exempt from the old package removal because of technical limitations.
+
 .. _`How is Copr pronounced?`:
 
 .. rubric:: How is Copr pronounced? :ref:`¶ <How is Copr pronounced?>`