Explain limitations of arbitrary values in bg-size and bg-position class groups in docs

[dcastil]

While working on #300 I realized that I only allow using arbitrary values in the class groups bg-size and bg-position with a label (e.g. length: and url: respectively).

The reason for it is that it is difficult to figure out whether something is a background-position or background-size with a simple regex since both allow similar-looking values.

I should explain these limitations of tailwind-merge in the docs to make it clear that this is the case and why.

[github-actions]

This was addressed in release v2.0.0.