docs/expensive-migrations: Revise a bit for clarity.
docs/expensive-migrations: Revise a bit for clarity.

Borrows some language from the paragraph in the changelog that
points to it.

timabbott/zulip,rht/zulip,synicalsyntax/zulip,timabbott/zulip,zulip/zulip,jackrzhang/zulip,brockwhittaker/zulip,shubhamdhama/zulip,eeshangarg/zulip,rishig/zulip,shubhamdhama/zulip,rht/zulip,rht/zulip,rishig/zulip,zulip/zulip,shubhamdhama/zulip,brainwane/zulip,rht/zulip,punchagan/zulip,brainwane/zulip,andersk/zulip,eeshangarg/zulip,mahim97/zulip,mahim97/zulip,rishig/zulip,Galexrt/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,timabbott/zulip,rht/zulip,eeshangarg/zulip,synicalsyntax/zulip,rht/zulip,hackerkid/zulip,synicalsyntax/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,jackrzhang/zulip,punchagan/zulip,timabbott/zulip,tommyip/zulip,jackrzhang/zulip,shubhamdhama/zulip,punchagan/zulip,kou/zulip,showell/zulip,brainwane/zulip,dhcrzf/zulip,zulip/zulip,synicalsyntax/zulip,mahim97/zulip,timabbott/zulip,brockwhittaker/zulip,rishig/zulip,shubhamdhama/zulip,dhcrzf/zulip,tommyip/zulip,jackrzhang/zulip,zulip/zulip,kou/zulip,timabbott/zulip,shubhamdhama/zulip,kou/zulip,kou/zulip,rishig/zulip,brainwane/zulip,synicalsyntax/zulip,eeshangarg/zulip,tommyip/zulip,Galexrt/zulip,synicalsyntax/zulip,andersk/zulip,zulip/zulip,andersk/zulip,Galexrt/zulip,zulip/zulip,kou/zulip,rishig/zulip,showell/zulip,eeshangarg/zulip,brainwane/zulip,punchagan/zulip,dhcrzf/zulip,kou/zulip,tommyip/zulip,timabbott/zulip,brockwhittaker/zulip,brockwhittaker/zulip,tommyip/zulip,synicalsyntax/zulip,rht/zulip,andersk/zulip,dhcrzf/zulip,andersk/zulip,rishig/zulip,showell/zulip,brockwhittaker/zulip,showell/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,Galexrt/zulip,shubhamdhama/zulip,zulip/zulip,Galexrt/zulip,hackerkid/zulip,dhcrzf/zulip,jackrzhang/zulip,showell/zulip,kou/zulip,jackrzhang/zulip,tommyip/zulip,hackerkid/zulip,brockwhittaker/zulip,tommyip/zulip,Galexrt/zulip,mahim97/zulip,dhcrzf/zulip,mahim97/zulip,jackrzhang/zulip,Galexrt/zulip,brainwane/zulip,dhcrzf/zulip,hackerkid/zulip,showell/zulip,mahim97/zulip,brainwane/zulip,andersk/zulip