Add note clarifying font interpretation

What kind of documentation issue is this?

Unclear or confusing documentation

Affected page

https://toga.beeware.org/en/stable/reference/api/resources/font/#notes

Describe the issue

As discussed in #2159, the note about font usage on iOS and macOS is misleading. It's not that iOS and macOS can't use variant fonts - it's that iOS and macOS will only use variants that are actually in a font file.

Suggested improvement

We should replace the note about iOS/macOS font usage with the following:

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Some platforms allow the use of font weights and variants that aren't explicitly provided by altering the rendering of a normal font (e.g., using a thick pen to render a normal font to render a fake bold, or applying a skew to render a fake italic). Toga only guarantees that the font faces, variants and weights that are actually defined in a font file will be available for use.

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