Docs: Improve documentation for `have_posts()`, `have_comments()`, and their...

Docs: Improve documentation for `have_posts()`, `have_comments()`, and their `WP_Query` counterparts.

Props stevenlinx.
Fixes #50610.
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git-svn-id: https://core.svn.wordpress.org/trunk@48175 1a063a9b-81f0-0310-95a4-ce76da25c4cd