bpf, docs: Move sentence about returning R0 to abi.rst

As discussed at LSF/MM/BPF, the sentence about using R0 for returning
values from calls is part of the calling convention and belongs in
abi.rst.  Any further additions or clarifications to this text are left
for future patches on abi.rst.  The current patch is simply to unblock
progression of instruction-set.rst to a standard.

In contrast, the restriction of register numbers to the range 0-10
is untouched, left in the instruction-set.rst definition of the
src_reg and dst_reg fields.
Signed-off-by: Dave Thaler <dthaler1968@googlemail.com>
Link: https://lore.kernel.org/r/20240517153445.3914-1-dthaler1968@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/standardization/abi.rst

@@ -23,3 +23,6 @@ The BPF calling convention is defined as:
 
 R0 - R5 are scratch registers and BPF programs needs to spill/fill them if
 necessary across calls.
+
+The BPF program needs to store the return value into register R0 before doing an
+``EXIT``.

Documentation/bpf/standardization/instruction-set.rst

@@ -476,9 +476,6 @@ the jump instruction.  Thus 'PC += 1' skips execution of the next
 instruction if it's a basic instruction or results in undefined behavior
 if the next instruction is a 128-bit wide instruction.
 
-The BPF program needs to store the return value into register R0 before doing an
-``EXIT``.
-
 Example:
 
 ``{JSGE, X, JMP32}`` means::