Documentation for `null` should mention `default`

[Zoybean]

I was looking for null-related commands in the book and (without knowing in advance what to search for) did not find anything about default until after searching for "null" on the nushell github and finding a stray comment mentioning the default command as the equivalent for rust's unwrap_or. It would be great to mention the available commands that manipulate possibly-null values in the section about null. I'm still not sure if I'm missing other relevant commands.