Document meta-merge behavior of config

[plexus]

When processing the #kaocha/v1 reader macro we merge the user's config into the default config using meta-merge. This means that you can add ^:prepend, :^append, and ^:replace metadata to influence how the merging should happen.

For example, user-configured plugins are added onto the list of default plugins [:kaocha.plugin/randomize :kaocha.plugin/filter :kaocha.plugin/capture-output], rather than replacing them. If you want to use a different variant of the filter plugin then you could use :plugins ^:replace [...] instead.

We do mention meta-merge in the docs, but only when used explicitly with a #meta-merge reader tag. There's also one example of a ^:replace in the cucumber test/doc for filter, but presented without much comment.