[m-dev.] for review: document pragma import & nondet pragma c_code

[Zoltan Somogyi]

> The compiler actually allows either `shared_code' or `duplicate_code'
> as alternatives to `common_code' -- `shared_code' results in smaller
> code, `duplicate_code' results in faster but larger code, while with
> `common_code' the compiler does its best to make the appropriate
> time-space trade-off.  Is it worth documenting this?

It must be documented, because it affects the semantics of any "static"
declarations in the C code; you don't want such declarations to be duplicated
by the compiler without the user's consent.

Zoltan.