Be clearer in docs about COPY/ADD dirs

COPY/ADD just copies the contents of dirs, not dirs themselves.
This PR tries to clear that up in the docs.

Closes #8775

Signed-off-by: Doug Davis <dug@us.ibm.com>
directory -> the directory?

Not sure if it should be "contents *are* copied", "contents *is* copied" or "content (singular) *is* copied". I'll leave that to the doc maintainers :smile: