[docs] Update the documention of exclusive range operators.

The operational semantic of the exclusive range operators is defined in
the Xbase API. The docummentation of SARL is updated for clarify this
operational semantic. The source of the explanation is the Xtext issue
[443258](https://bugs.eclipse.org/bugs/show_bug.cgi?id=443258).

see #190

Signed-off-by: sgalland <galland@arakhne.org>