Docs: Clarify description of remove_all_filters() method in WP_Hook class

[SirLouen]

The phrase "current filter" is misleading, it sounds like it refers to whichever filter is currently executing, which is a different concept entirely. The remove_all_filters method simply removes all callbacks from that instance. This probably was copied from previous methods which refer to a specific hook, so this feels a little copy glitch

Trac ticket: https://core.trac.wordpress.org/ticket/64717

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