bpf, docs: Add a setion to explain the basic instruction encoding

The eBPF instruction set document does not currently document the basic
instruction encoding.  Add a section to do that.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220103183556.41040-2-hch@lst.de

Documentation/bpf/instruction-set.rst

@@ -19,8 +19,22 @@ The eBPF calling convention is defined as:
 R0 - R5 are scratch registers and eBPF programs needs to spill/fill them if
 necessary across calls.
 
+Instruction encoding
+====================
+
+eBPF uses 64-bit instructions with the following encoding:
+
+ =============  =======  ===============  ====================  ============
+ 32 bits (MSB)  16 bits  4 bits           4 bits                8 bits (LSB)
+ =============  =======  ===============  ====================  ============
+ immediate      offset   source register  destination register  opcode
+ =============  =======  ===============  ====================  ============
+
+Note that most instructions do not use all of the fields.
+Unused fields shall be cleared to zero.
+
 Instruction classes
-===================
+-------------------
 
 The three LSB bits of the 'opcode' field store the instruction class: