document line height variable for custom font size

[marcospgp]

the docs should mention that if you want to add a custom font size you should also add a corresponding line height

for example, I wanted to have text-md be equivalent to text-base, so I needed to add:

@theme {
...

  --text-md: var(--text-base);
  --text-md--line-height: var(--text-base--line-height);
}

the docs don't mention this in the theme variables doc. one has to look at the font size documentation and infer that because it applies two variables, one must also define these two for a custom font size.