Deep equality including keys is not documented

[planttheidea]

When doing a deep equality comparison, the deep equality of keys as well as values are used for Map. This is intentional, as it allows for comparisons like this:

const mapA = new Map([[{ foo: 'bar' }, { bar: 'baz' }]]);
const mapB = new Map([[{ foo: 'bar' }, { bar: 'baz' }]]);

deepEqual(mapA, mapB); // true

Since the spec of Map lookups use SameValueZero for keys, it is not obvious that this is the case. The documentation should call out this feature and its reasoning so developers are aware.

Originally called out in this PR by @g-traub .