message-model: Add `zulipterminal` to sent_by_human clients.
message-model: Add `zulipterminal` to sent_by_human clients.

Tweaked by tabbott to also more fully document this feature.

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