Tweaks to ViewportTexture's Documentation

[Mickeon]

Yeah, sure, why not.

• Add an actual concrete example in the leading description.
• There's a minor yet notable difference between saying "create a ViewportTexture" and "get a ViewportTexture". The latter is correct, the former makes it sound like it's a new one, every time.
  • Closes Confused about creating ViewportTexture in code godot-docs#7576.
• Added a note about the lag spike with huge ViewportTextures.
  • Closes (?) Instancing relatively large Viewport Textures consistently causes frame drops. godot-docs#4084.
• viewport_path's description has changed to mention what it refers to (get_local_scene) and to simplify the additional note.

[Mickeon]

Poking to have a small look among the other spring-cleaning class reference changes. It may be possible to backport to 4.2 and 4.1 as well.

[Mickeon]

If I could give a thumbs up to this I would, I assure you.

[akien-mga]

Thanks!