Add "implementation details" section to schema

[dcbaker]

As discussed here: #108, some CPS fields have specific implementation details or workarounds in mind, such as the need to add -rpath-link with GNU's ld.bfd. We could add special section of the description to document those description to assist implementers.

[mwoehlke-kitware]

I'm still not 100% convinced we need to get that specific. OTOH, there are possibly areas where that level of detail wouldn't be unwelcome.

It would also be good to have somewhere to clarify that the various [*_]requires are intended to be disjoint. (Mentioning that in the high level description of all such attributes gets rather verbose, not to mention being somewhat hard to maintain.)

[nickelpro]

"Rationale" is an extremely useful discussion to have, if not directly in the schema than somewhere. It is immensely helpful to implementers to know the motivations and history behind various corners of technical specs.

This is akin to the non-normative notes in the C++ spec. Is it important to know that 9.3.4.3(7) [dcl.ref] is the reference collapse rule? No but the spec adds it as a note anyway so implementers can say "Oh ya this is reference collapse, ok, I know what this is for".

[mwoehlke-kitware]

"Rationale" is an extremely useful discussion to have

Okay, that's a more palatable way to frame it. Although I'm still not sure it should be our job to teach people how to use specific linkers.