Document expectations for bounds on various C-language types

[rwatson]

The CHERI C Programming Guide should more clearly document what minimum expectations exist for bounds and permissions on allocations arising from C-language definitions -- e.g., globals, locals / dynamic stack allocations, thread-local storage, etc. This is distinct from expectations for standard library interfaces (see #9).

@ruben-arm, @PeterSewell, @yury-khrustalev, @brooksdavis