Docs error: merge applies from content to theme and not viceversa

[mtrebron]

In the section https://github.com/plone/plone.app.theming/blob/master/docs/index.rst#merge-and-copy the sentence below is wrong.

<merge /> will add the contents of the named attribute(s) in the theme to the value(s) of any existing attributes with the same name(s) in the content, separated by whitespace. It is mainly used to merge CSS classes.

the opposite is true, as documented here: http://docs.diazo.org/en/latest/basic.html#merge