Docs: Correct the WP_Block_Parser_Frame::leading_html_start type.

The property was documented as `int`, but the constructor declares `$leading_html_start` as an optional parameter defaulting to `null` and assigns it straight through, so the property holds `null` whenever no offset is supplied. This is unlike the neighboring `$prev_offset`, which is coalesced to a computed integer and therefore is accurately documented as `int`. The property is now documented as `int|null`, matching the constructor's own parameter documentation.

Developed in WordPress#7881.
Follow-up to r56048, r61504.

Props marian1, mukesh27.
Fixes #62561.


git-svn-id: https://develop.svn.wordpress.org/trunk@63444 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-includes/class-wp-block-parser-frame.php

@@ -51,7 +51,7 @@ class WP_Block_Parser_Frame {
 	 * Byte offset into document where leading HTML before token starts
 	 *
 	 * @since 5.0.0
-	 * @var int
+	 * @var int|null
 	 */
 	public $leading_html_start;