Clarify create_from_image() usage

[KoBeWi]

create_from_image() should be always used to create ImageTexture and can no longer be used on existing ImageTexture. I don't know the internals exactly, but judging from the old descriptions there is no point in reusing ImageTexture as create_from_image() is the most expensive part of it anyway.

Resolves #63025
Closes #63131

EDIT:
Turns out there is some use case-case for updating ImageTexture, which is when you have multiple copies of it and want to update all of them, so I updated the PR to expose set_image().

[Zylann]

update was clear, the problem was that I couldn't notice create_from_image was static #63131 (comment)

[akien-mga]

Thanks!