Describe difference between font size and bbox

[radarhere]

Resolves #7802

The user in the issue thinks that the documentation could use further detail about the transition from font.getsize() to font.getbbox().

I've amended the deprecations and release notes documentation with the following

Previously, the size methods returned a height that included the vertical offset of the text, while the new bbox methods explicitly distinguish this as a top offset.

While here, I also noticed that some of the example code didn't use the font declared earlier, so I've fixed that.