Add acceptance test for method outcomes

This was written as part of updating the documentation to reflect the current
state.

Ref: https://dev.cfengine.com/issues/6538
"Commands... are"  all plural, but "a promise" singular.  Ugly phrasing.
Also, the "by default" is so placed that it could be read as meaning the command returns nonzero by default.
"A command that returns nonzero is, by default, a promise not kept."