Add cross references

Signed-off-by: Dave Thaler <[email protected]>

Author: dthaler

isa/kernel.org/instruction-set.rst

@@ -506,10 +506,10 @@ BPF_IMM | BPF_DW | BPF_LD  0x18    0x06  dst = mva(map_by_idx(imm)) + next_imm
 
 where
 
-* map_by_fd(fd) means to convert a 32-bit POSIX file descriptor into an address of a map object
+* map_by_fd(fd) means to convert a 32-bit POSIX file descriptor into an address of a map object (see `Map objects`_)
 * map_by_index(index) means to convert a 32-bit index into an address of a map object
 * mva(map) gets the address of the memory region expressed by a given map object
-* variable_addr(id) gets the address of a variable with a given id
+* variable_addr(id) gets the address of a variable (see `Variables`_) with a given id
 * code_addr(offset) gets the address of the instruction at a specified relative offset in units of 64-bit blocks
 * the 'imm type' can be used by disassemblers for display
 * the 'dst type' can be used for verification and JIT compilation purposes