changed initStatic?Dict() return type to const ZSTD_?Dict*

changed initStatic?Dict() return type to const ZSTD_?Dict*

ZSTD_create?Dict() is required to produce a ?Dict* return type
because `free()` does not accept a `const type*` argument.
If it wasn't for this restriction, I would have preferred to create a `const ?Dict*` object
to emphasize the fact that, once created, a dictionary never changes
(hence can be shared concurrently until the end of its lifetime).

There is no such limitation with initStatic?Dict() :
as stated in the doc, there is no corresponding free() function,
since `workspace` is provided, hence allocated, externally,
it can only be free() externally.

Which means, ZSTD_initStatic?Dict() can return a `const ZSTD_?Dict*` pointer.

Tested with `make all`, to catch initStatic's users,
which, incidentally, also updated zstd.h documentation.