Ignore docs could be clarified by adding a list of allowed keywords

[jcollum-nutrien]

From https://github.com/gotwarlost/istanbul/blob/master/ignoring-code-for-coverage.md:

A coverage skip hint looks like /* istanbul ignore <word>[non-word] [optional-docs] */

Except there's no list of what word and optional-docs can be nor any link to where that list can be found.