docs: explicitly specify mail.use-{tls,ssl} is mutually exclusive

[aldy505]

I don't remember if it's on Discord or on Github issues, but someone had a problem setting this up and although it's Django's behavior, not everyone code in Django.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.01%. Comparing base (5bd6cd3) to head (59b9902).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3368   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files           3        3           
  Lines         203      203           
=======================================
  Hits          201      201           
  Misses          2        2           

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[joshuarli]

thanks!