Document Param trait

[kennykerr]

More can be said about the Param trait, but this at least provides a start by including some preliminary documentation.

• The IntoParam trait is renamed Param to improve comprehension.
• The Param trait is now unhidden and provides a basic description of its purpose and use.
• Some internal supporting types have been renamed, even though they remain hidden, to further clarify their purpose and meaning.

Fixes: #1499