Thanks! I have a documentation revisit somewhere in queue with T8636, but the master vs off-grid-in-Tahoe timing ended up a little rough.

Since the config option already has a documentation link, we could skip the redundant one here -- I think it's okay to just point users at "cluster.mailers", and then the help there can guide them toward configuration. I'll probably beef up the cluster.mailers help a bit in an upcoming change.

(In the general case, you can generate links with pht(...) by using %s with phutil_tag(...), or use PhabricatorEnv::getDoclink(). The theoretical advantage of getDoclink() is that it could some day be configurable to point at a local documentation source rather than the upstream documentation, in case installs want to fork the documentation.)

Because this is routine during setup, consider a more first-time-setup focused name with less emphasis on the technical detail that the option is named cluster.mailers, e.g. just "Mailers Not Configured" + "You haven't configured mailers yet, so Phabricator won't be able to send outbound mail or receive inbound mail. To configure mailers..." or something along those lines. My expectation is that everyone will hit this during setup now, since we no longer default to sendmail (which was probably a bad default in the first place).

(We should probably also add this to the "Quick Start" Guide thing or maybe throw that away, but it's a bit off the beaten path and I can deal with that during the followup documentation pass.)