Docs: Indicate that `get_posts()` can take a query string in addition to an array of query vars.

A string is supported by virtue of `wp_parse_args()` being used on the supplied arguments. While a string is not currently passed to `get_posts()` in core, a significant number of plugins are doing so. So in addition to documenting actual ecosystem usage, this also adds a test to ensure that supplying a query string continues to be supported in the future.

Developed in https://github.com/WordPress/wordpress-develop/pull/11179

Follow-up to r11528.

Props rodrigosprimo, westonruter, shailu25, mukesh27.
See #64224, #10047.
Fixes #64813.


git-svn-id: https://develop.svn.wordpress.org/trunk@62041 602fd350-edb4-49c9-b593-d223f7449a82