docs: document disabling native RNF logs

[huytdps13400]

Summary

• document the existing iOS $RNFEnableLogs opt-out
• document the existing Android RNF_enableLogs opt-out
• clarify that native rebuilds are required and application console.log output is unaffected

Verification

• cd docs && yarn typecheck
• cd docs && yarn build
• git diff --check

Closes #309.

[huytdps13400]

Superseded by #347, which documents the same supported build flags in the repository's dedicated Logging guide and includes the more complete verification record. Closing this duplicate so there is only one active PR for #309.