cbcollect_info: single- vs multi-node-diag options

There was a change made to cbcollect_info in 4.6.4, 5.0.1 and 5.1.0 (MB-25679), where the default behaviour was changed to only collect diag from one node, and the -single-node-diag option was replaced with -multi-node-diag to allow full-cluster diag to be collected if desired.

The documentation for 4.6, 5.0, and 5.1 is currently incorrect for versions 4.6.4, 4.6.5, 5.0.1, and all of 5.1.x.