billing: Add comment explaining race conditions in initial subscription.
billing: Add comment explaining race conditions in initial subscription.

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