Clarify what `global.font.size` affects

[jcfilben]

Received feedback in slack that the global.font.size prop documentation is misleading. The way it is written makes it sounds like it is controlling the font size for most of the application when in reality the font size for most components is set in other areas of the theme.

Link to slack thread: https://grommet.slack.com/archives/C09QQEG69/p1672269258235529 Link to global.font.size documentation: https://v2.grommet.io/globaltheme#global.font.size