Write new documentation for this module.  Ensure that it formats
correctly in "cmake --help-module FindBoost" output.  Show the basic
form of calling find_package(Boost).  Document all result variables,
input variables, and cache variables appropriately grouped together.
Explain the search process and how it re-runs when changes are made.
Explain the difference between finding headers/libraries versus finding
a "Boost CMake" package configuraiton file.

Drop the emphasis on Boost_ADDITIONAL_VERSIONS because the
implementation should predict most future versions instead.