include: Clarify variable access scope for included file

The wording "Commands in the file are processed immediately as if they
were written in place of the include command" sounds as if some kind of
macro replacement is performed.  This is not accurate.  Update the
wording to describe behavior of the included code in terms of the
variable access scope.
Merge topic 'doc-include-clarify' into next

e934b1f include: Clarify variable access scope for included file