docs: Clarification for type which calls itself "JSON" but actually cannot be.

[mt-inside]

What this PR does / why we need it: Clicking through the docs from EnvoyProxy, looking to customize a gateway Deployment or Service, you land on these docs. The type of value is given as k8s type JSON, but for what I assume are complicated api-machinery deserialization quirks, cannot acutally be a JSON string, which is how I read it. Adding clarification to that.

Ie This is fine, and is not "JSON" in the simple meaning

value:
  foo: bar

Note that the above is first-class, ie NOT

value: |
  foo: bar

And this is actually rejected

value: '{ "foo": "bar" }'

[zirain]

@mt-inside the doc is generated from /api, need to change there and run make gen-check.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.92%. Comparing base (2314729) to head (976b177). Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4117      +/-   ##
==========================================
- Coverage   67.93%   67.92%   -0.02%     
==========================================
  Files         187      187              
  Lines       23018    23018              
==========================================
- Hits        15638    15634       -4     
- Misses       6265     6268       +3     
- Partials     1115     1116       +1     

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[mt-inside]

@zirain Done!