Update c-call section to mention that char is signed

We should mention in the manual that c-call:char is signed. c-call:unsigned-char is, obviously, unsigned. This is an important detail because in C, char can be either, and you have to use signed char or unsigned char to be sure to specify the signedness of a char.

But see also cmucl#251; when that lands, we'll need to update the manual again to make a note of that.