Document sourcemaps upload and inject --ignore semantics #2990
Description
The --ignore parameter is not very well documented currently, and is therefore being used incorrectly not only by some external users, but also in some places we use --ignore internally.
Therefore, we should provide better documentation for how --ignore works.
We originally planned to change --ignore's behavior in #2951, but since doing so proved to be less trivial than hoped, and perhaps also undesirable, given the many edge cases we would be breaking, documentation is likely the better path here.