Need to clarify / explain how options merging for arrays works

[jobara]

The documentation for options merging doesn't describe the default behaviour for how arrays are merged.

Currently it seems that arrays will merge by position, that is merging [1, 2, 3] onto ["a", "b", "c", "d"] would produce [1, 2, 3, "d"]. It also appears that if you want a shorter array that the original you'd need to specify a merge policy of "replace" for that option path.

see channel discussion: https://botbot.me/freenode/fluid-work/2017-07-04/?msg=88145239&page=1.

Additional context or notes

Originally filed as FLUID-6164 by @jobara on July 4, 2017.