Add comment on differences with upstream apachectl, no functional

change.

apachectl.sh

@@ -15,6 +15,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+###
+### NOTE: This is a replacement version of the "apachectl" script with
+### some differences in behaviour to the version distributed with
+### Apache httpd.  Please read the apachectl(8) man page for more
+### information.
+###
+
 if [ "x$1" = "x-k" ]; then
     shift
 fi