complete SSLListener doc

[WindSoilder]

Trying to make SSLListener doc complete, which is referred to #540 .

[njsmith]

The Jenkins failure is MagicStack/immutables#7, nothing you need to worry about.

I'm going to add a period at the end of the sentence, to satisfy my nitpicking and also trigger a test re-run... hopefully this time it will pass and we can merge :-).

[codecov]

Codecov Report

Merging #581 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #581   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files          89       89           
  Lines       10628    10628           
  Branches      747      747           
=======================================
  Hits        10551    10551           
  Misses         59       59           
  Partials       18       18

Impacted Files	Coverage Δ
trio/_ssl.py	100% <ø> (ø)	⬆️

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[njsmith]

Closing/open to trigger a re-test, now that #583 is merged.

[njsmith]

Okay, finally! Sorry for those delays.

And... welcome! 🎉 🎂 No pressure, but if you'd like to keep contributing then we'd love to have you, so I'm sending you a github invite now. You can read more about this in our contributing documentation.