Clarify "concrete type" and make type category descriptions consistent (closes #1780) (#1798)

* Clarify "concrete type" and make type category descriptions consistent (closes #1780)

* markdown fixup
Co-authored-by: NJonathan Wakely <cxx@kayari.org>

* Elaborated that we can't generally add comparison operators to C structs
Co-authored-by: NSergey Zubkov <cubbi@cubbi.com>
Co-authored-by: NJonathan Wakely <cxx@kayari.org>