[cache] Adding description for a zero cache timeout

[john-bodley]

This PR explicitly calls out in the forms that a timeout of 0 indicates that the cache never expires.

to: @mistercrunch

[codecov-io]

Codecov Report

Merging #5354 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5354   +/-   ##
=======================================
  Coverage   61.33%   61.33%           
=======================================
  Files         369      369           
  Lines       23488    23488           
  Branches     2713     2713           
=======================================
  Hits        14407    14407           
  Misses       9069     9069           
  Partials       12       12

Impacted Files	Coverage Δ
superset/views/core.py	72.96% <ø> (ø)	⬆️
superset/connectors/druid/views.py	68.24% <ø> (ø)	⬆️
superset/connectors/sqla/views.py	70.47% <ø> (ø)	⬆️

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[mistercrunch]

LGTM - (optional) could be good to refactor the common text into a single place