Improve the documentation for the config-service.cache.enabled configuration property.

[klboke]

What's the purpose of this PR

Improve the documentation for the config-service.cache.enabled configuration property.

Motivation

• We encountered a situation in our production environment where, when caching was enabled, we were unable to retrieve the latest configuration due to a case mismatch between apollo.cluster keys.

Possible solutions

If we want to enable in-memory caching and ensure that the loading behavior remains consistent with when it is not enabled, could we simply convert the cache key of Guava Cache to lowercase? Would this cause any additional problems? I'm not entirely sure. One thing to consider is that the code already handles namespaces in a similar way, so is this new issue consistent with that?

[codecov]

Codecov Report

Merging #4810 (bf5e812) into master (a441b6b) will not change coverage.
The diff coverage is n/a.

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##             master    #4810   +/-   ##
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  Coverage     47.23%   47.23%           
  Complexity     1660     1660           
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  Files           346      346           
  Lines         10685    10685           
  Branches       1065     1065           
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  Hits           5047     5047           
  Misses         5329     5329           
  Partials        309      309           

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[nobodyiam]

If we want to enable in-memory caching and ensure that the loading behavior remains consistent with when it is not enabled, could we simply convert the cache key of Guava Cache to lowercase?

It's actually a known issue as is also recorded in the apollo project. There was an attempt to fix this issue #3542 but not continued. I think we could still consider fixing it if there is an easier way.

[klboke]

The worst part is that MySQL was not strictly set to match case sensitivity from the beginning, so now we have to do compatibility processing. I think we can merge this PR first, As for the compatibility processing issue, I will look into it later.

[nobodyiam]

LGTM