Custom transform option not supported

**I'm submitting a bug report**

**Current behavior:**
[Docs tell us](https://github.com/babel/babel-loader/blob/master/src/cache.js#L138-L159) that we can pass in a custom `transform` function option but it's not actually supported like it was [earlier](https://github.com/babel/babel-loader/blob/4b3bafa4859774458327a585d69a3c346a7652d6/src/fs-cache.js), right now it always uses the imported one.

**Expected/desired behavior:**
Either support the custom `transform` function or remove it from the docs.

Start by inspecting src/cache.js at the documented lines and compare its transform handling with the earlier src/fs-cache.js version linked in the issue. Confirm whether the custom transform option is supported; done means either the option works as documented or the documentation no longer promises it.

Written by the indexing model from the issue text.