Improve inline docs for wp_dropdown_pages() - mention support for get_pages() arguments

[birgire]

It's not clear from the documentation that wp_dropdown_pages() also supports get_pages() input arguments.

Here are two ways to fix that:

1) For comparison we have the following docblock line in wp_list_pages() src:

@see get_pages() 

2) We also have this docblock line in wp_page_menu() src:

*     Optional. Arguments to generate a page menu. See wp_list_pages() for additional arguments.

where we look at the wp_list_pages() reference for comparison.

[birgire]

43355.diff​ adds a get_pages() references in the docblock of wp_dropdown_pages().

[SergeyBiryukov]

In 42721:

Docs: Add a reference to get_pages() arguments in wp_dropdown_pages() and wp_list_pages(), as they both use the function internally.

Props birgire.
Fixes #43355.