Clarified comment for purpose of GatherVcfs

[rickymagner]

Description

This very very short PR clarifies the purpose of the GatherVcfs tool, changing the phrase "totally discrete" to the more accurate "nonoverlapping." Compare with the Picard documentation here to see this is the intended purpose. The reason this confused me a bit was that technically you could have two vcfs with discrete sets of intervals which do overlap (e.g. chr1:100-101 + chr2:100-101 and chr1:100-101 + chr3:100-101), which certainly should not be used as input into this tool. Changing it here should eventually change the corresponding text on the GATK documentation page as well, once it's incorporated into GATK.

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