Improve formatting of inline documentation for `get_pages()`.

Adds missing default values, corrects several argument types, backtick-escapes argument names used in descriptions.

See #28298.

Built from https://develop.svn.wordpress.org/trunk@30725


git-svn-id: http://core.svn.wordpress.org/trunk@30715 1a063a9b-81f0-0310-95a4-ce76da25c4cd