Clarify reasons for physics processes

[danielsjf]

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I noticed some conflicting wordings on reading this page.

I also updated the section on the physics process callback. This is quite well explained in another section of the documentation so I summarized that explanation and referred to it. The main reasoning (why use a different callback) was not directly clear from the previous explanation.

[danielsjf]

@fabriceci let me know if you have further comments.

[danielsjf]

@fabriceci Done. Is it not possible from your side to squash merge?

[akien-mga]

Is it not possible from your side to squash merge?

It's possible for the docs repo, but not for the main engine repo (goes against our policies for a clean and consistent Git history). So sometimes we forget about the option for godot-docs :)