Document the use of empty parentheses in equations

[c3d]

In the current implementation, it is possible to insert empty parentheses in algebraic equations, that will "steal" a value from the stack when evaluating. This actually looks like a nice feature, but it would need to be documented.

Examples:

• 'A' 'B' '()+()' EVAL gives 'A+B'.
• X 'sin()' EVAL gives ``sin X`.

This is an extension to traditional RPL.