Add docs regarding plugin timeouts and multicached

[argaen]

Closes #388
Closes #372

[codecov]

Codecov Report

Merging #438 into master will increase coverage by 0.54%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #438      +/-   ##
==========================================
+ Coverage   99.23%   99.78%   +0.54%     
==========================================
  Files          10       10              
  Lines         917      917              
  Branches      101      101              
==========================================
+ Hits          910      915       +5     
+ Misses          3        2       -1     
+ Partials        4        0       -4

Impacted Files	Coverage Δ
aiocache/decorators.py	100% <ø> (ø)	⬆️
aiocache/backends/redis.py	100% <0%> (+3.2%)	⬆️

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