help: Correct broken link and clarify email gateway integration.
help: Correct broken link and clarify email gateway integration.

This helps make it clear that this feature isn't supported on all
Zulip installations.

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