Docs: Clarify the usage of `null` for `auto_update_{$type}` filter.

The value is internally used to detect whether nothing has hooked into this filter.

Props audrasjb, pbiron, johnbillion, SergeyBiryukov.
Merges [48909] to the 5.5 branch.
Fixes #50848.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48678 1a063a9b-81f0-0310-95a4-ce76da25c4cd