api: Fix incorrectly documented event types.
api: Fix incorrectly documented event types.

It's actually "subscription" and "message" (neither is plural).

While we're at it, we should also remove the "pointer" event type,
since that's of generally low interest.

andersk/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,synicalsyntax/zulip,timabbott/zulip,rht/zulip,kou/zulip,brainwane/zulip,rishig/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,zulip/zulip,kou/zulip,jackrzhang/zulip,synicalsyntax/zulip,dhcrzf/zulip,eeshangarg/zulip,rht/zulip,timabbott/zulip,timabbott/zulip,rishig/zulip,tommyip/zulip,punchagan/zulip,rishig/zulip,hackerkid/zulip,eeshangarg/zulip,tommyip/zulip,brainwane/zulip,zulip/zulip,synicalsyntax/zulip,jackrzhang/zulip,synicalsyntax/zulip,timabbott/zulip,dhcrzf/zulip,showell/zulip,showell/zulip,zulip/zulip,rht/zulip,punchagan/zulip,tommyip/zulip,rht/zulip,shubhamdhama/zulip,rishig/zulip,rishig/zulip,hackerkid/zulip,kou/zulip,jackrzhang/zulip,rht/zulip,punchagan/zulip,tommyip/zulip,timabbott/zulip,rishig/zulip,eeshangarg/zulip,timabbott/zulip,dhcrzf/zulip,eeshangarg/zulip,dhcrzf/zulip,shubhamdhama/zulip,shubhamdhama/zulip,shubhamdhama/zulip,jackrzhang/zulip,kou/zulip,andersk/zulip,jackrzhang/zulip,hackerkid/zulip,showell/zulip,showell/zulip,zulip/zulip,brainwane/zulip,dhcrzf/zulip,punchagan/zulip,shubhamdhama/zulip,andersk/zulip,andersk/zulip,synicalsyntax/zulip,rht/zulip,brainwane/zulip,showell/zulip,brainwane/zulip,dhcrzf/zulip,hackerkid/zulip,hackerkid/zulip,synicalsyntax/zulip,andersk/zulip,jackrzhang/zulip,tommyip/zulip,eeshangarg/zulip,rishig/zulip,synicalsyntax/zulip,brainwane/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,jackrzhang/zulip,tommyip/zulip,punchagan/zulip,showell/zulip,rht/zulip,andersk/zulip,tommyip/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,shubhamdhama/zulip,kou/zulip,dhcrzf/zulip,zulip/zulip,timabbott/zulip