[docs] Clarify variable-width integer (VBR) encoding example.

[vsapsai]

Summary

Show that the bit chunks are placed starting at the least significant
bit. Select a different number, so the bit chunks have different values
and it is more obvious where they are in the encoded result.

[vsapsai]

Tested the actual numbers with

TEST(BitstreamWriterTest, emitDocSample) {
  SmallVector<char> Buffer;
  BitstreamWriter W(Buffer);
  W.EmitVBR(30, 4);
  W.FlushToWord();
  EXPECT_EQ((SmallVector<char>{0b0011'1110, 0, 0, 0}), Buffer);
}

in BitstreamWriterTest.cpp. Don't think it is worth keeping as we don't enforce the docs to be in sync with the unit tests.

[vsapsai]

Thanks for the review!

llvm/docs/BitCodeFormat.rst

 ^^^^^^^^^^^^^^^^^^^^^^^
 
 Variable-width integer (VBR) values encode values of arbitrary size, optimizing
 for the case where the values are small.  Given a 4-bit VBR field, any 3-bit
 value (0 through 7) is encoded directly, with the high bit set to zero.  Values
 larger than N-1 bits emit their bits in a series of N-1 bit chunks, where all
 but the last set the high bit.
 
-For example, the value 27 (0x1B) is encoded as 1011 0011 when emitted as a vbr4
-value.  The first set of four bits indicates the value 3 (011) with a
-continuation piece (indicated by a high bit of 1).  The next word indicates a
-value of 24 (011 << 3) with no continuation.  The sum (3+24) yields the value
-27.
+For example, the value 30 (0x1E) is encoded as 62 (0b0011'1110) when emitted as
+a vbr4 value.  The first set of four bits starting from the least significant
+indicates the value 6 (110) with a continuation piece (indicated by a high bit
+of 1).  The next set of four bits indicates a value of 24 (011 << 3) with no
+continuation.  The sum (6+24) yields the value 30.
 
 .. _char6-encoded value:
 
 6-bit characters
 ^^^^^^^^^^^^^^^^
 
 6-bit characters encode common characters into a fixed 6-bit field.  They
 represent the following characters with the following 6-bit values: