Spec update for Procedures vs Export/Extern declarations

Spec 0.95 has some conflation between Procedure definitions, declarations for external functions, and restrictions for defining procedures that can be called by external functions.

Separate these out.

Likewise, I'd leave this description of the linkage specifiers as-is, relying on detail in the interop chapter to talk about uses of the syntax that will be rejected by the compiler.
(The user doesn't have to know whether it's bison or some later stage of translation that's issuing the complaint.  In fact, it's better if the parser proper accepts all possible productions, because "Syntax error." tells you precious little about what you did wrong....)