Rationale for this change

While implementing Variable-size Binary View Layout (thanks @ariesdevil !) in apache/arrow-rs#5481 it was not 100% clear if the inlined string was zero padded.

@bkietz noted that

The spec does say "padded with zero" https://github.com/apache/arrow/blob/main/docs/source/format/Columnar.rst?plain=1#L384 but it could be repeated in the surrounding paragraph. In any case, padded with zero is definitely the intent

    * Short strings, length <= 12
      | Bytes 0-3  | Bytes 4-15                            |
      |------------|---------------------------------------|
      | length     | data (padded with 0)                  |

What changes are included in this PR?

Add a sentence in the surrounding text to make it clear the inlined strings values are zero padded

Note I do not think this is a specification change (and therefore doesn't need a vote on the mailing list) as the spec already specifies the padding is zero (in the diagram). This simply clarifies the text to emphasize this point for ease of understanding

Are these changes tested?

Are there any user-facing changes?