Complete the docs for Quaternion

[HexagonNico]

Added documentation for some methods in the Quaternion class where it was missing.

[AThousandShips]

I'm not sure about linking to Wikipedia here, the link is not guaranteed to be preserved, and we don't do it for the other ones here, I'd suggest providing an explanation here instead, as the equations aren't very complicated

[AThousandShips]

Especially considering this is the code, which is trivial to explain:

Quaternion Quaternion::log() const {
	Quaternion src = *this;
	Vector3 src_v = src.get_axis() * src.get_angle();
	return Quaternion(src_v.x, src_v.y, src_v.z, 0);
}

Quaternion Quaternion::exp() const {
	Quaternion src = *this;
	Vector3 src_v = Vector3(src.x, src.y, src.z);
	real_t theta = src_v.length();
	src_v = src_v.normalized();
	if (theta < CMP_EPSILON || !src_v.is_normalized()) {
		return Quaternion(0, 0, 0, 1);
	}
	return Quaternion(src_v, theta);
}

And at least I can't work out what the method is doing from the Wiki but the code is very clear

[HexagonNico]

I thought it could be explained with a link since I saw it is done in Vector3. I changed the docs to explain the code better.

[fire]

The use of exponential and log of quaternion is that if you randomly pick a value it will be on the rotation sphere, which then you can use then extract into a quaternion rotation.

Any quaternion that isn't on the rotation sphere is not a rotation.

http://www.jp.square-enix.com/tech/library/pdf/Implementation_Details_20170730_SIGGRAPH%2717_David.pdf

https://web.archive.org/web/20170825184056/http://number-none.com/product/Understanding%20Slerp,%20Then%20Not%20Using%20It/

See the review of the three methods of rotation interpolation by Jonathan Blow:

See also https://web.mit.edu/2.998/www/QuaternionReport1.pdf for an exhaustive textbook.

[akien-mga]

Looks good! Could you squash the commits? See PR workflow for instructions.

While doing so, please amend the resulting commit message to be more explicit (e.g. like the PR title).

[HexagonNico]

Commits squashed!

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.2.2.