CNV getbayesianhetcoverage: tag doc, give example command and interval_list, massage summary, prioritize modes

[sooheelee]

• Food for thought: The snpIntervals should also accept a VCF of snps.
• For future iteration: should probably mention where the Bayesian math comes in. I assume at the inference.

[sooheelee]

Ignore this one. See instead https://github.com/broadinstitute/gatk-protected/pull/1125