markdown: Clarify url parameter of "add_a" helper.
markdown: Clarify url parameter of "add_a" helper.

andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip