api docs: Document historical changes to typing API.
api docs: Document historical changes to typing API.

Along with other recent changes, this fixes #13286.

zulip/zulip,hackerkid/zulip,brainwane/zulip,rht/zulip,shubhamdhama/zulip,synicalsyntax/zulip,andersk/zulip,andersk/zulip,andersk/zulip,showell/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,hackerkid/zulip,showell/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,shubhamdhama/zulip,showell/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,shubhamdhama/zulip,showell/zulip,timabbott/zulip,showell/zulip,timabbott/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,timabbott/zulip,timabbott/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,shubhamdhama/zulip,eeshangarg/zulip,brainwane/zulip,showell/zulip,punchagan/zulip,shubhamdhama/zulip,brainwane/zulip,zulip/zulip,brainwane/zulip,rht/zulip,rht/zulip,shubhamdhama/zulip,eeshangarg/zulip,timabbott/zulip,kou/zulip,kou/zulip,timabbott/zulip,timabbott/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,punchagan/zulip,andersk/zulip,brainwane/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,rht/zulip,kou/zulip,synicalsyntax/zulip,zulip/zulip,synicalsyntax/zulip,brainwane/zulip,rht/zulip,synicalsyntax/zulip,synicalsyntax/zulip