Update `opcode.md` to match the latest implementation
MODIFY
doc/internal/opcode.md
doc/internal/opcode.md
@@ -7,19 +7,23 @@ reducing the memory consumption of mruby VM.
# Instructions
Instructions are bytes. There can be 256 instructions. Currently, we
-have 94 instructions. Instructions can take 0 to 3 operands.
+have 106 instructions. Instructions can take 0 to 3 operands.
## operands
The size of operands can be either 8bits, 16bits or 24bits.
-In the table.1 below, the second field describes the size (and
-sign) of operands.
+In the table.1 below, the second field describes the size
+of operands.
- B: 8bit
- S: 16bit
-- sS: signed 16bit
- W: 24bit
+If the first and second operands are of type `B` (8bits), they may be
+extended to 16bits by the operand extension instruction immediately
+preceding them.
+See also `OP_EXT1`, `OP_EXT2` and `OP_EXT3`.
+
## table.1 Instruction Table
| Instruction Name | Operand type | Semantics |

Update `opcode.md` to match the latest implementation