Docs: Correct type and improve description for the `$fields` parameter of...

Docs: Correct type and improve description for the `$fields` parameter of `WP_Query::parse_query()`.

Props ocean90, palmiak, johannadevos, noisysocks.
Fixes #44305.
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git-svn-id: https://core.svn.wordpress.org/trunk@48325 1a063a9b-81f0-0310-95a4-ce76da25c4cd