Docs could be clearer about .pth files that work in editable install scenarios

[debonte]

Some of our docs say things like "use .pth files instead of import hooks" which is confusing since import hooks are set up via .pth files. I'm not sure what the simplest language would be, but the concept we're trying to get across is that we work with .pth files that only contain a list of paths, without executable code.

Places to update:

• https://microsoft.github.io/pyright/#/import-resolution?id=editable-installs
• https://code.visualstudio.com/docs/python/editing#_importresolvefailure