Documentation is not clear about how to use source maps in custom transformers

🐛 Bug Report

Documentation is not clear about how to use source maps in custom transformers. Additionally, if you don’t pass any source maps, jest will report a failed assertion in a random position in the original not transformed source code.

To Reproduce

Steps to reproduce the behavior:

1. Visit Custom transformers documentation page.
2. Create a custom transform following the documentation.
3. Create a test that fails.
4. See the jest output pointing at a random position in the original source code.

Expected behavior

Inform how the source map should be returned by a custom transformer. If the code was transformed and the source map was not provided, don’t just assume the lines and columns will match.

Link to repl or repo (highly encouraged)

https://repl.it/@fcostarodrigo/JestCustomTransformIssue

I am using CoffeeScript just as an example.

envinfo

  System:
    OS: Linux 5.4 Debian GNU/Linux 9 (stretch) 9 (stretch)
    CPU: (4) x64 Intel(R) Xeon(R) CPU @ 2.30GHz
  Binaries:
    Node: 12.18.2 - /usr/local/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.5 - /usr/local/bin/npm
  npmPackages:
    jest: ^24.9.0 => 24.9.0 

[SimenB]

#12566 (hopefully clearer now, also #12407)

[Mskat]

Can I grab it?