vop docs

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+VOPs:
+
+%prologue      on x86, this does nothing. On PowerPC, at the start of
+               each word that calls a subroutine, we store the link
+               register in r0, then push r0 on the C stack.
+
+%call-label    on PowerPC, uses near calling convention, where the
+               caller pushes the return address.
+
+%dispatch      compile a piece of code that jumps to an offset in a
+               jump table indexed by an integer. The jump table must immediately follow this VOP.
+
+%slot          the untagged object is in vop-out-1, the tagged slot
+               number is in vop-in-1.
+
+%fast-slot     the tagged object is in vop-out-1, the pointer offset is
+               in vop-in-1. the offset already takes the type tag into
+               account, so its just one instruction to load.
+
+%set-slot      the new value is vop-in-1, the object is vop-in-2, and
+               the slot number is vop-in-3.
+
+%fast-set-slot the new value is vop-in-1, the object is vop-in-2, and
+               the slot offset is vop-in-3.
+               the offset already takes the type tag into account, so
+               it's just one instruction to load.
+
+%parameters    ignored on x86.
+
+%parameter     ignored on x86.
+
+%unbox         an unboxer function takes a value from the data stack
+               and converts it into a C value.
+               
+%box           a boxer function takes a C value as a parameter and
+               converts into a Factor value, and pushes it on the data
+               stack.
+
+               on x86, C functions return integers in EAX.
+
+%box-float     on x86, C functions return floats on the FP stack.
+
+%box-double    on x86, C functions return doubles on the FP stack.
+
+%cleanup       ignored on PowerPC.
+
+               on x86, in the cdecl ABI, the caller must pop input
+               parameters off the C stack. In stdcall, the callee does
+               it, so this node is not used in that case.