minor clarification for matching pairs (documentation)

[tpapp]

In retrospect it is pretty obvious, but I needed to look up the associativity of => to figure out how to match Pair(x,y) expressions, ie

@match :(a => b) begin
    (x_ => y_) => (x,y)         # need parentheses
    _ => :unmatched
end

where x_ => y_ => (x,y) won't work because it is parsed as x_ => (y_ => (x,y)). Thanks for the great package, if you consider this issue too trivial just close it.

[MikeInnes]

Not too trivial at all. It might be useful to have an examples.jl file or something with common patterns like this – feel free to open a PR for that.