test_subs: Add comments about num_events.
test_subs: Add comments about num_events.

This is a bit complex, because many events not directly related
to what we are testing are also sent during these operations, hence
these comments.

zulip/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,rht/zulip,punchagan/zulip,rht/zulip,zulip/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,kou/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,kou/zulip