Documentation for new_admin_email_content hook should contain a list instead of headings

[dd32]

As reported in ​https://github.com/WordPress/Documentation-Issue-Tracker/issues/1208 and then ​https://github.com/WordPress/phpdoc-parser/issues/236 the PHPdoc for the new_admin_email_content filter includes some markdown headers.

While an argument could be made that this should be handled on the parser side, I think support for headings is valuable for some code such as used in ​WP_HTML_Tag_Processor.

Trac ticket: https://core.trac.wordpress.org/ticket/60262#ticket

[peterwilsoncc]

In 57289:

Docs: Format new_admin_email_content placeholders as a list.

Format the email placeholders for the new_admin_email_content hook as a list for clarity and to avoid parsing errors in docblock consumers.

Props dd32, shooper, stevenlinx.
Fixes #60262.

Merged in r57289 / ​https://github.com/WordPress/wordpress-develop/commit/8dee7819693c3b0c5bc98eb2d15b2a37452bc62a