Fix HUD image (waypoint) docs

[appgurueu]

Fixes #14158.

Relevant code:

minetest/src/client/hud.cpp

Lines 451 to 456 in 2766c70

	v2s32 dstsize(imgsize.Width * e->scale.X * m_scale_factor,
	imgsize.Height * e->scale.Y * m_scale_factor);
	if (e->scale.X < 0)
	dstsize.X = m_screensize.X * (e->scale.X * -0.01);
	if (e->scale.Y < 0)
	dstsize.Y = m_screensize.Y * (e->scale.Y * -0.01);

I've also updated the test in devtest to check this.

The same applies to images, so I fixed the docs there too.