Allow defining callable setters for switches and settings

[rytilahti]

This fixes a logic flaw that made it impossible to define a setter callable for switches and settings.

Includes a couple of unrelated doc fixes to get the tests pass.

[codecov-commenter]

Codecov Report

Merging #1504 (a582904) into master (26987b5) will decrease coverage by 0.01%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master    #1504      +/-   ##
==========================================
- Coverage   82.62%   82.61%   -0.02%     
==========================================
  Files         142      142              
  Lines       14078    14080       +2     
  Branches     3390     3392       +2     
==========================================
  Hits        11632    11632              
  Misses       2223     2223              
- Partials      223      225       +2     

Impacted Files	Coverage Δ
.../integrations/airpurifier/dmaker/airfresh_t2017.py	96.15% <ø> (ø)
...io/integrations/vacuum/dreame/dreamevacuum_miot.py	74.38% <ø> (ø)
miio/device.py	56.00% <14.28%> (-0.65%)	⬇️

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