tar: add comment to link04.at test

* tests/link04.at: Add explanatory comment at head.

tests/link04.at

@@ -20,6 +20,12 @@
 
 # written by Paul Eggert
 
+# Make sure that tar -c correctly handles the case where a file is
+# encountered multiple times, even though it has a link count of 1.
+# This can occur when "tar -c FOO FOO" is used; it can also occur when
+# "tar -ch FOO" is used, if FOO contains symbolic links that point to
+# the same file.
+
 AT_SETUP([link count is 1 but multiple occurrences])
 AT_KEYWORDS([hardlinks link04])