Document site cleanup behaviour.

As per #2014, document the fact that the destination directory is
'cleaned up', such that "obsolete" files (files which do not have a
known input Page, Post or StaticFile) are deleted, when a site is
rebuilt.

Resolves #2014.
[ci skip]
Clarifying wording of the destination directory warning.

At some point, it should also be clarified whether "folder" or
"directory" is the correct way to refer to that concept.
Merge pull request #2016 from Jashank/destination-warning