document typedef guard macros, etc for ptraddr_t etc

[brooksdavis]

We currently declare ptraddr_t and define _PTRADDR_T_DECLARED as a macro at the same time to indicate that it has been declared. We need to document this and need add __SIZEOF_* and any related macros for new types.

[nsz-arm]

those macros sound like perprocessor api between compiler and libc which would be nice to document, but not necessary in a language spec.

however ptraddr_t is a c language extension that violates current iso c rules (not reserved name for stddef.h) so user code may need to know about it too not just the libc. i think such extension needs a preprocessor check to indicate the language variant and version (like __STDC_VERSION__ >= 199901L check for iso c99 compatibility). but it's not clear what that language is until there is a complete cheri c language spec.