doc(macros): typo and other improvements

[robstoll]

I was thinking about revising the section about getValue in addition in this PR but are unsure if I interpret the docs correctly (unfortunately I don't have time to setup a dotty project and scastie cannot run macros as they are defined in the same file).
The text is very vague expression containing constants. I guess getValue works if the expression is constant, correct? In other words '(2).getValue and '(1+1).getValue both yield Some(2). Correct? If so then I'll create another PR improving the corresponding section

[nicolasstucki]

In general, getValue will only get a value if the source contains explicitly the value. But literal constants may get constant folded before macro expansion, hence '(1+1) will be constant folded to '(2) before the evaluation of the macro.