Document BoostConfig.cmake?

The documentation for your version of BoostConfig.cmake seems a little sparse: https://github.com/boostorg/cmake/blob/7f5336b3bf8067bb40da4e8b9940c133271e938a/config/BoostConfig.cmake#L5

CMake 3.30 deprecated [FindBoost](https://cmake.org/cmake/help/latest/module/FindBoost.html), saying:

> Changed in version 3.30: This module is available only if policy [`CMP0167`](https://cmake.org/cmake/help/latest/policy/CMP0167.html#policy:CMP0167) is not set to NEW. Port projects to upstream Boost's `BoostConfig.cmake` package configuration file, for which `find_package(Boost)` now searches.

Sounds reasonable, except they didn't mention that there are two of those, and , neither of which appears to be fully documented in its own right, seemingly depending on the FindBoost documentation to describe how things are meant to work.

Fortunately, the CMake folks don't seem the type to actually remove deprecated stuff in a hurry, especially not the documentation, but its not exactly clear what the user is actually supposed to before flipping this policy *especially* if they want to make sure their software works with both the "boost_install" and "cmake" variants.

P.S. I sure wish I could open this against *both* repositories. Darn you, github!