MINOR: [Docs] Clarify inlined strings in VariableLengthStringView i…

…s padded with `0` (#40512)

### 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?

Authored-by: Andrew Lamb <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

docs/source/format/Columnar.rst

@@ -393,7 +393,8 @@ length of the string and can be used to determine how the rest of the view
 should be interpreted.
 
 In the short string case the string's bytes are inlined — stored inside the
-view itself, in the twelve bytes which follow the length.
+view itself, in the twelve bytes which follow the length. Any remaining bytes
+after the string itself are padded with `0`.
 
 In the long string case, a buffer index indicates which data buffer
 stores the data bytes and an offset indicates where in that buffer the