Improve instructions for using a b2-built or installed Boost from CMake

People are finding it difficult to use a b2-built or installed Boost from CMake, and this issue

https://github.com/boostorg/cmake/issues/77

has turned into an unofficial support forum for that.

There's a suggestion in the last comment that the official documentation could be improved to explain this.

Start with the discussion in boostorg/cmake#77, especially its last comment, to identify the missing guidance. Then locate the relevant Boost CMake documentation and update it to explain using a b2-built or installed Boost from CMake; done means the common usage path is covered without relying on the issue as a support forum.

Written by the indexing model from the issue text.