[Chicken-hackers] Request to edit manual Foreign type specifiers

[Kon Lovett]

Hi Folks,

I would like to edit the "Foreign type specifiers" manual section so  
pointer entries follow the "optional attribute" convention introduced  
by '[nonnull-] c-string*'. I will also add a general note about the  
pointer passing/return #f <-> NULL conversion.

In keeping with the spirit above specifiers like "unsigned-integer"  
will become "[unsigned-] integer".

Let me know if any issues,

Thank you,
Kon