docs: add migration hint for alias resolving

[timozander]

While migrating to Vite 2, I found myself having trouble with my alias config. The Migration guide did not explicitly mention the change to @rollup/plugin-alias and the implications of that.

PR aims at clarifying that. 😄

[yyx990803]

This seems to have bitten a lot of people - the change is in 2.0 it's a direct replacement, so if the replacement doesn't end with a slash, /@foo/bar will be resolved as /.../some-special-dirbar (missing the slash in between).

The regex works, but you can do without a regex by ensuring an ending slash:

alias: { '/@foo/': path.resolve(__dirname, 'some-special-dir') + '/' }

[timozander]

Thanks, didn't realize that. I changed the text accordingly