doc,src: clarify that one napi_env is for each specific module
One napi_env is for each specific module in one context.
Related: https://github.com/nodejs/node/pull/28682 Fixes: https://github.com/nodejs/node/issues/31003
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines