docs: include RN dependencies flag for iOS source builds

[sorinc03]

Summary

• Add RCT_USE_RN_DEP=0 alongside RCT_USE_PREBUILT_RNCORE=0 in the React Native 0.84 iOS source-build instructions
• Update the Legacy Architecture opt-in command so both precompiled iOS artifacts are disabled

Fixes #5065

Test Plan

• git diff --check

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[cipolleschi]

Hi there, thanks for the contributions, but this should not be merged.
The RNDependencies do not care about new arch/old arch or legacy hermes/hermes v1.
They are third party libraries built independently from React Native, so users can still benefit from using the precompiled binaries even if they use some different setups than the recommended ones.

You almost never want to build RNDependencies from source.