Clarify how Query::get_component works

[alice-i-cecile]

Objective

• Users were confused about the data that Query::get_component can access.

what's the point of the get_component(_mut) methods on Query, do they completely ignore the Q and F generic parameters on the query itself?

(from Discord

Solution

• Explain that the query is limited to declared component accesses.
• Also leave a small note about the importance of requesting mutable access.

[Aurora2500]

I think a better example would be great too, since the one used right now is identical to that of the method Query::get, and from my perspective doesn't show what its point is.

Maybe a small change would be to make Character a WorldQuery and have a new Health component be part of this WorldQuery, explaining this relation between the two and how get_component deals with it.

[Selene-Amanita]

I don't think that's true for get_component_mut, it returns a MissingWriteAccess in that case, there is a doc test for that case: https://docs.rs/bevy/latest/bevy/ecs/system/enum.QueryComponentError.html#variant.MissingWriteAccess