XSMULTI developer experience (DX) points

[mohawk2]

XSMULTI is a powerful feature (I may be biased), but there are a couple of subtle gotchas that could be warned about:

• if the XSBUILD option is present but XSMULTI is not, nothing happens and there is no warning
• if the XSBUILD value misses out the e.g. xs layer and just has values like lib/..., it is wrong and there is no warning

I believe at least a "gotchas" paragraph on either or both the XS* docs would help this, but warnings seem like a decent idea too, in line with the README.pod ones.