Add documentation for babel sourceMap

[richtera]

Add documentation for babel sourceMap.

[davidsneighbour]

This is ambiguous: Sourcemap settings inside of the babel.config.js are being ignored other than inputSourceMap: true which defaults to true. -- would inputSourceMap: false be ignored or not? if the value is irrelevant to the ignoring, let's remove the : true part of the sentence.

[richtera]

Somehow the docs for the sourceMap "external" support in both js.Build and Babel didn't end up in the docs when the PR was merged. I might be adding the docs in the wrong place.