Misleading information in section "Rational types"

[phoe]

cl-cookbook/numbers.md

Lines 71 to 82 in fd22cd8

	### Rational types
	Rational numbers of type [`ratio`][ratio] consist of two `bignum`s, the
	numerator and denominator. Both can therefore be arbitrarily large:
	~~~lisp
	* (/ (1+ (expt 2 100)) (expt 2 100))
	1267650600228229401496703205377/1267650600228229401496703205376
	~~~
	It is a subtype of the [`rational`][rational] class, along with
	[`integer`][integer].

The type bignum is specified to be exactly (and integer (not fixnum)), which, along with the first line of this paragraph, would then mean that 1/2 is not a valid ratio because neither 1 and 2 are bignums.

I'd rather say something similar to the specification: that numerator and denominator are both integers, their GCD is 1, and numerator must be greater than 1.

I'd also mention that the conversion between ratio and integer is automatic so the programmer doesn't need to worry about overflows or underflows or imprecise computation, but also means that it's easily possible to reach bignum arithmetic and therefore get computation that is slower and consing.