[NFC][RFC][TableGen] Improve the comment about variable len encoder

[0x59616e]

Summary

When, for example, using "ascend" operator, I was wondering whether
the bit order of the dag argument will be reversed in ascend order
as well.

This patch clarifies it.

[myhsu]

This is pretty neat. Thank you!

[0x59616e]

Thanks @myhsu ;)

llvm/include/llvm/Target/Target.td

 def ins;
 
 /// variable_ops definition - Mark this instruction as taking a variable number
 /// of operands.
 def variable_ops;
 
 /// variable-length instruction encoding utilities.
 /// The `ascend` operator should be used like this:
-///     (ascend 0b0000, 0b1111)
+///     (ascend 0b0010, 0b1101)
 /// Which represent a seqence of encoding fragments placing from LSB to MSB.
-/// Thus, in this case the final encoding will be 0b11110000.
+/// Thus, in this case the final encoding will be 0b1101_0010.
 /// The arguments for `ascend` can either be `bits` or another DAG.
 def ascend;
 /// In addition, we can use `descend` to describe an encoding that places
 /// its arguments (i.e. encoding fragments) from MSB to LSB. For instance:
-///     (descend 0b0000, 0b1111)
-/// This results in an encoding of 0b00001111.
+///     (descend 0b0010, 0b1101)
+/// This results in an encoding of 0b0010_1101.
 def descend;
 /// The `operand` operator should be used like this:
 ///     (operand "$src", 4)
 /// Which represents a 4-bit encoding for an instruction operand named `$src`.
 def operand;
 /// Similar to `operand`, we can reference only part of the operand's encoding:
 ///     (slice "$src", 6, 8)
 ///     (slice "$src", 8, 6)