Documents some of the weirdness of __curPos and the or keyword.
This does not fit well into any existing section for either of
them, though the use of or as a quasi-operator is mentioned in
the section on operators.
Addresses https://git.lix.systems/lix-project/lix/issues/353
Change-Id: I7c906c8368843dca6944e8b22573b6d201cd9a76