Document that the socket sendall method is not task-safe

[njsmith]

See gh-291

[codecov]

Codecov Report

Merging #292 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
- Coverage   99.18%   99.16%   -0.03%     
==========================================
  Files          79       79              
  Lines        9658     9658              
  Branches      686      686              
==========================================
- Hits         9579     9577       -2     
- Misses         62       63       +1     
- Partials       17       18       +1

Impacted Files	Coverage Δ
trio/_util.py	90.52% <0%> (-2.11%)	⬇️

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