Docs: Improve documentation for `format_for_editor()` and the...

Docs: Improve documentation for `format_for_editor()` and the `'the_editor_content'` filter it is hooked to.

Props AramZS for initial patch.
Fixes #34866.
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git-svn-id: https://core.svn.wordpress.org/trunk@35868 1a063a9b-81f0-0310-95a4-ce76da25c4cd