Add C# XML documentation to core C# math types

[aaronfranke]

Wow, this took about 20 hours! It seems that now about a fifth of the total lines are documentation, judging by cat * | wc -l. So far I've only done math types, I will do another PR for the rest of the files. This is part of #39703.

A lot of this documentation was taken from the core documentation, but while making this I found many areas where the grammar of the documentation could be improved, and also I discovered a few parts of the documentation that are just simply wrong.

While making this, I also added a few minor changes, including adding missing braces, simplifying/fixing code in a few places, and renaming arguments to be more descriptive. I also removed Bounce and Reflect from Vector2i because they are not methods that make sense in an integer context. I suppose I could split this into another PR, but it's much easier to... not do that.

For both of the above paragraphs, I plan to do a pass over the core files in future PRs, fixing minor API inconsistencies and updating the documentation to be more descriptive, by taking changes from this PR.

[neikeq]

Thanks a lot for this huge work!

[neikeq]

I don't really like the idea of enforcing braces in C# where the opener goes on a new line. We don't have to follow the C++ code style here. Or do we actually enforce braces in the C# code style and I forgot about it?

This is not a blocker, but I would prefer without it if possible (unless it's from our C# code style and I forgot).

[aaronfranke]

The C# style guide doesn't say that braces should always be enforced, but it does say how they should be placed, all examples have braces, and I would totally be in favor of adding this to the style guide. It is more readable in my opinion, and I think being consistent with the C++ code is a good thing.

[akien-mga]

2 lines vs 4 lines for a short if is a tough sell with the brace style used for C# though.

I don't think the C# and C++ code styles have to be consistent per se, but I'm not opposed to it either.

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/inside-a-program/coding-conventions shows both styles in their examples but predominantly the one with braces though.

[aaronfranke]

Ok, I changed some of the parts of the code back to no braces, specifically, the parts in AABB/Rect2 with repeated "return false;" statements, since I think those parts of the code need it the least.

However, I kept other instances of braces, since I think it's more readable and should be the default choice.

[aaronfranke]

I did a lot of force pushes recently because I was adding the documentation to core and while doing so I noticed a few problems. However, I think the documentation in this PR is good now.

[akien-mga]

Thanks!