Child and cluster docs

/to @bnoordhuis @schoonology

This series of docs attempts to describe aspects of cluster that I found most difficult to work with, either because the docs were wrong, misleading, absent, or just incomprehensible. I have verified the docs mostly through code inspection, and occaisonally through small samples.

I found that the behaviour they describe is not implementation detail, but necessary for the successful use of the public API. Things that look like they were not intended to be part of the API (the execArgv option to setupMaster, for example) are still undocumented.

Changes should also be merged to master, but I have applied to v0.10 to be of immediate use to cluster users.