Add documentation description about "visionOS" in OS::get_name

[OzelotVanilla]

Mentioned in another PR of mine #108034 (comment), copied and rephased comment from there.

Found the doc doc/classes/OS.xml, lacks the "visionOS" one.

godot/doc/classes/OS.xml

Lines 372 to 383 in 9a39760

	<method name="get_name" qualifiers="const">
	<return type="String" />
	<description>
	Returns the name of the host platform.
	- On Windows, this is [code]"Windows"[/code].
	- On macOS, this is [code]"macOS"[/code].
	- On Linux-based operating systems, this is [code]"Linux"[/code].
	- On BSD-based operating systems, this is [code]"FreeBSD"[/code], [code]"NetBSD"[/code], [code]"OpenBSD"[/code], or [code]"BSD"[/code] as a fallback.
	- On Android, this is [code]"Android"[/code].
	- On iOS, this is [code]"iOS"[/code].
	- On Web, this is [code]"Web"[/code].
	[b]Note:[/b] Custom builds of the engine may support additional platforms, such as consoles, possibly returning other names.

which is implemented already:

godot/platform/visionos/os_visionos.mm

Lines 48 to 50 in 9a39760

	String OS_VisionOS::get_name() const {
	return "visionOS";
	}