docs: Update client.md for the request notes change.
docs: Update client.md for the request notes change.

`request.client` is no longer valid since the ZulipRequestNotes change.
This update the documentation to reflect that. And it also makes it
recommend `check_send_webhook_message` in favor of
`check_send_stream_message`.

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