Documentation tweak: Built-in type support wasn't obvious to me

Hi. Thanks for creating this!

In relatively casual usage of Hive for a demo, it wasn't obvious to me what types have built-in support. I just guessed that uInt8List and String were probably supported, and it worked, so I went with it. I assume less common basic types like, say, DateTime don't have built-in support, but I'm actually not all that sure. I did poke around the API documentation to see if I could find a normative specification, but I couldn't. Sorry if I missed something obvious.

**SUGGESTED FIX**

The four methods [Box | LazyBox].[put | get] should say where this is specified. If the types with built-in support aren't already listed somewhere, perhaps the top-level package documentation would be a good place.

Start with the API documentation for Box and LazyBox, focusing on the put and get methods mentioned in the issue. Determine where built-in type support is specified, or whether the top-level package documentation needs a list; done means the supported types and their authoritative documentation location are clear to users.

Written by the indexing model from the issue text.