Improve inline documentation for `Walker_Nav_Menu`, `Walker_Nav_Menu_Edit`, `Walker_Nav_Menu_Checklist`.

Includes previously undocumented parameters.

See #23406.

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git-svn-id: http://core.svn.wordpress.org/trunk@25560 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Merge branch 'master' of https://github.com/WordPress/WordPress