Docs: Reference the documentation of the new `navigation_widgets_format`...

Docs: Reference the documentation of the new `navigation_widgets_format` filter instead of repeating it.

Follow-up to [48349].

Props johnbillion.
Fixes #48170.

Built from https://develop.svn.wordpress.org/trunk@48388


git-svn-id: https://core.svn.wordpress.org/trunk@48157 1a063a9b-81f0-0310-95a4-ce76da25c4cd