[Libretro] Adds some core options description + minor changes

[bslenul]

• Changed the "Allow Naomi Service Buttons" option to "Allow Arcade Service Buttons", since AFAICT they work for other arcade systems as well.
• Show Naomi free play option only for Naomi games. Unless it works for other systems as well? I tried Dolphin Blue and it wasn't free play.
• Show 32MB hack and VMU sound options only for DC.
• Fixed the "Texture Upscaling Max. Filtered Size" option not being hidden properly ("off" was replaced by "1" a while ago: 4fe8e40#diff-d7895b9548aafcfabf4b7487c4c29a7dbe6b181aebc84d0fac8cb9cdc8722f28)
• Changed default position for VMUs B1/C1/D1, IMO it makes more sense than having them all in the top-left corner.
• Added multiplier in the internal resolution names. Do we really need all these res up to x20 btw? Seems a bit overkill 😅 There are a few weird ones also...
• Added some core options descriptions, always kinda hard for me since English isn't my native language, I'm always scared that it's not correct, concise and/or clear enough :/

Open to any suggestions of course!

[flyinghead]

Yes, x20 upscaling is insane, and will crash in per-pixel.
Needless to say I didn't make this list.