fix docs for Entity::from_row

[zacryol]

Objective

The docs state that Entity::from_row creates an entity with a generation of 1, which was the case previously. With Entity internals being reworked in 0.17 however, this is no longer the case, as the generation used is now EntityGeneration::FIRST, currently equivalent to 0.

Solution

This PR changes the docs to state an "unspecified" generation to allow further changes to implementation without needing another rewrite, as the docs rather consistently refer to Entity, EntityRow, and EntityGeneration as being opaque identifiers.

[mockersf]

https://docs.rs/bevy/latest/bevy/ecs/entity/struct.EntityGeneration.html#associatedconstant.FIRST is public and could be linked to maybe?

[ElliottjPierce]

Ooh, nice catch. I forgot to update that.

IMO, we have two options:

• Change docs to say "unspecified" as is done here.
• Specify that it is EntityGeneration::FIRST and rename to first_id_at_row or something to reflect it in the name.

Personally, I would prefer the first option, the current implementation, because that gives us the most flexibility and because if anyone cares about what generation they get, they will use from_row_and_generation instead anyway.

[Victoronz]

I think marking it unspecified is fine for now.