fix: arc_default_max (zfs_arc_max) on Linux matches FreeBSD as of zfs-2.3.0

[JCBird1012]

This change was made in #15437 and has been active as of zfs-2.3.0.

I'm not sure the convention for dealing with behavior changes between ZFS versions in docs, so I left the pre-v2.3.0 behavior listed, and notated it as such. Let me know if I should remove it.

[Copilot]

Pull Request Overview

Updates the module parameters documentation to reflect the change in default ARC max sizing on Linux as of OpenZFS 2.3.0, aligning it with FreeBSD.

• Document post-2.3.0 behavior: Linux matches FreeBSD (max ARC = max(all_memory - 1GB, 5/8 of memory))
• Retain and label pre-2.3.0 behavior for Linux

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[Copilot]

The standalone italic line 'prior to v2.3.0' breaks the bullet list into two separate lists and is stylistically inconsistent. For clarity and proper rendering, consider using a Sphinx versionchanged directive and explicit labels, e.g., '.. versionchanged:: 2.3.0' describing the change, and restructure as a single section with labeled subsections or a nested list (e.g., 'As of 2.3.0:' and 'Prior to 2.3.0:'). Also, prefer 'Prior to 2.3.0:' (capitalized, no 'v') to match typical version notation.

Suggested change

	* **Linux** and **FreeBSD**: the larger of ``all_system_memory - 1GB`` and ``5/8 × all_system_memory``
	*prior to v2.3.0*
	* **Linux**: 1/2 of system memory
	* **FreeBSD**: the larger of ``all_system_memory - 1GB`` and ``5/8 × all_system_memory``
	* As of 2.3.0:
	* **Linux** and **FreeBSD**: the larger of ``all_system_memory - 1GB`` and ``5/8 × all_system_memory``
	.. versionchanged:: 2.3.0
	Prior to 2.3.0:
	* **Linux**: 1/2 of system memory
	* **FreeBSD**: the larger of ``all_system_memory - 1GB`` and ``5/8 × all_system_memory``

[JCBird1012]

I like this suggested change in some ways (it's technically the correct Sphinx way), but not others -

IMO, current behavior should always be listed first, with previous behavior denoted below, so I'm not sure how I feel about it.