Allow to update `doc/internal/opcode.md` file mechanically from `include/mruby/ops.h` file.
At the same time, the `doc/internal/opcode.md` file has been updated.
Consecutive hyphens at the end of the table were removed as they would have created noise as markdown flavours.
Transforms the value of a splat inside a return statement (similar
to an array). For example, `return *nil` should return `nil.to_a`,
while `return *1` should return `[1]`
Yukihiro "Matz" Matsumoto