docs: correct site package autoconfiguration

[nightcityblade]

Summary

Document the current site-package-path autoconfiguration: Pyrefly derives it from filtered sys.path entries rather than site.getsitepackages().

Fixes #4222

Test Plan

• yarn build (passed; only existing broken-anchor warnings were reported)
• Changed MDX block verified against Prettier output
• env PATH=/Users/shensheng/.cargo/bin:$PATH python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema (passed)

AI Usage

This documentation correction was prepared with an AI coding agent and reviewed against the current interpreter-query implementation.

[meta-cla]

Hi @nightcityblade!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!