Entity: Document upsert methods

[brandonroberts]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Feature request
[X] Documentation issue or request

What is the current behavior?

Expected behavior:

Minimal reproduction of the problem with instructions:

Version of affected browser(s),operating system(s), npm, node and ngrx:

Other information:

Add documentation for the newly added upsertOne and upsertMany methods once #550 lands