Document strip-common-prefix and strip-prefix option

[szokeasaurusrex]

@andreiborza discovered that the strip-common-prefix option operates on each path passed to the sourcemaps upload command independently.

Also investigate strip-prefix; perhaps list it as an alternative for use cases where both paths should be considered together (i.e. when only the common portion of the two paths should be stripped)

Verify that this is the case and document the behavior.