[config] clarifications around SUPERSET_WEBSERVER_TIMEOUT

[mistercrunch]

Closes #6836

[codecov-io]

Codecov Report

Merging #6840 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #6840   +/-   ##
=======================================
  Coverage   56.32%   56.32%           
=======================================
  Files         527      527           
  Lines       23528    23528           
  Branches     2782     2782           
=======================================
  Hits        13251    13251           
  Misses       9866     9866           
  Partials      411      411

Impacted Files	Coverage Δ
superset/config.py	93.67% <100%> (ø)	⬆️

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