Add some notes in changelog or docs about `site` after v20.0.0b1

[YouJiacheng]

There are lots of issues about site.py in legacy implementation.
But thing seems completely different in the second iteration of implementation, according to the source code, #2105, #2106 and #1421 (comment).
This could be confusing, e.g. https://stackoverflow.com/a/46071447 said "With virtualenvs getsitepackages is not available".
When I searched about this, there isn't any thing tell me the thing is differenet now, I only got numerous issues that said virtualenvs doesn't support site module well.
Related issues:
#228
#355
#434
#571
#587
#737
#1250 (link in this issue should be updated to https://github.com/pypa/virtualenv/blob/legacy/virtualenv_embedded/site.py)
They all said "there isn't site.getsitepackages".

[gaborbernat]

Please open a PR for it 👍