Restore :: in Makefile.am

The double-colon looked like a typo, but it's actually an obscure
feature. Rules with :: may appear multiple times and are run
independently of one another in the order they appear. The use of ::
for distclean-local was conventional, not accidental.

Add comments to indicate the intentional use of double-colon rules.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9210

tests/zfs-tests/include/Makefile.am

@@ -16,5 +16,6 @@ $(nodist_pkgdata_DATA): %: %.in
 		-e 's,@sysconfdir\@,$(sysconfdir),g' \
 		$< >'$@'
 
+# Double-colon rules are allowed; there are multiple independent definitions.
 distclean-local::
 	-$(RM) default.cfg

tests/zfs-tests/tests/functional/pyzfs/Makefile.am

@@ -14,5 +14,6 @@ $(pkgpyzfs_SCRIPTS):%:%.in
 		$< >'$@'
 	-chmod 775 $@
 
+# Double-colon rules are allowed; there are multiple independent definitions.
 distclean-local::
 	-$(RM) $(pkgpyzfs_SCRIPTS)