Clarify documentation for `enable` pseudo-variable

[gridbugs]

The documentation for variables says that for boolean variables the form name1+name2+name3:var is equivalent to name1:var & name2:var & name3:var but it's possible to write name1+name2:enable despite enable being the string "enable" or "disable". This change clarifies that enable isn't really a variable but syntactic sugar instead.

[rjbou]

test in #5642 updated