Add map support doc comments to Subset and NotSubset

[jedevc]

Summary

Updates the doc comments in Subset and NotSubset functions to include information about the map support added in https://github.com/stretchr/testify/pull/1178.

Changes

Simple doc-comment changes only.

Motivation

At the moment, it's not clear that the Subset family of functions supports maps as parameters, while the Contains family of functions above do and document it - this behavior makes it seem like Subset doesn't support maps.

Related issues

• https://github.com/stretchr/testify/issues/1173
• https://github.com/stretchr/testify/pull/1178