Missing documentation

For the latest version of the documentation (for nbformat 5.1.3), the following is either lacking or missing:

1. **nbformat.v4.upgrade** is not documented, but very useful for upgrading to the latest minor version of the v4 format. Notably, I was having the issue of adding new cells to a notebook using the 4.2 format, which made the notebook format invalid, because the different `v4.new_cell` methods adds a cell id which is incompatible with the <4.5 formats, and there is no way to specify a minor version when adding new cells.
2. **nbformat.v4.upgrade** returns the upgraded notebook whenever the notebook is modified, but returns `None` otherwise. Based on the least surprise principle, this is not the best strategy in my opinion. It would be better to always return the notebook, or at least to make this behavior explicit in the documentation.
3. The arguments of the **nbformat.validate** method are not currently documented, and some of them are not obvious.

Start by locating the documentation for nbformat.v4.upgrade and nbformat.validate, then compare it with their current APIs. Done means documenting upgrade, including its return behavior, and documenting the arguments of validate; the issue does not name specific files or tests.

Written by the indexing model from the issue text.