[boost] [parameter] Literate test, and other changes.

Hi, Looking at parameter's history there are three groups of unmerged changes: - "literate" tests extracted from the documentation (plus some small documentation changes) - Stephen Kelly's changes removing workarounds for old compilers - a warning fix - https://github.com/boostorg/parameter/commit/b5e57e0024ae51f5e706eff50e86023... I think they're all okay to merge, although I'm not sure about the literate tests, does anyone know why they weren't merged earlier? They don't show up in the test results, although maybe they're not meant to. Daniel