MINOR: [Docs] Clarify struct validity masking with 'hidden data' example (#49554)

### Rationale for this change

The current documentation contains a technical inconsistency in the validity bitmap values for column 4.

According to the Arrow specification:

> In Arrow, a dedicated buffer, known as the validity (or “null”) bitmap, is used alongside the data indicating whether each value in the array is null or not: a value of 1 means that the value is not-null (“valid”), whereas a value of 0 indicates that the value is null.

In the existing example, the validity bitmap for the third row of the variable-size binary child array is incorrectly set to 1, despite the row being null. This PR corrects that bit to 0 to align with the fixed-size primitive child array and the overall Arrow memory layout standards.

### What changes are included in this PR?

Updated the struct-diagram.svg

### Are these changes tested?

No

### Are there any user-facing changes?

No

Authored-by: Philipp Ucke <philippucke@googlemail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>

Author: pucke-dev

docs/source/format/Intro.rst

@@ -296,6 +296,13 @@ key is the field name and the child array its values. The field (key) is
 saved in the schema and the values of a specific field (key) are saved in
 the child array.
 
+Since child arrays are independent, Arrow does not enforce physical
+consistency between the struct's validity bitmap and those of it's children.
+Logically, a struct row is only valid if both the parent and the child
+bitmaps have a value of 1 for that slot (a logical AND operation).
+This allows for "hidden" data to exist in child arrays at null struct
+positions (see ``alice`` below).
+
 .. figure:: ./images/struct-diagram.svg
    :alt: Diagram is showing the difference between the struct data type
          presented in a Table and the data actually stored in computer