add boxes section to proposed-terminfo.md

doc/proposed-terminfo.md

@@ -8,9 +8,19 @@ The `blocks` capability indicates that block characters are drawn by the
 terminal itself, rather than through rendering the current font. This should
 cover at least:
 
-* The entirety of the [Block Elements](https://www.unicode.org/charts/PDF/U2580.pdf) Unicode block (U+2580--U+259F).
-* The entirety of the [Geometric Shapes](https://www.unicode.org/charts/PDF/U25A0.pdf) Unicode block (U+25A0--U+25FF).
-* The [Symbols for Legacy Computing](https://www.unicode.org/charts/PDF/U1FB00.pdf) Unicode block from U+1FB00 through U+1FBAF,
-   except U+1FB70--U+1FB7F.
+* The entirety of the [Block Elements](https://www.unicode.org/charts/PDF/U2580.pdf)
+   block (U+2580–U+259F).
+* The entirety of the [Geometric Shapes](https://www.unicode.org/charts/PDF/U25A0.pdf)
+   block (U+25A0–U+25FF).
+* The [Symbols for Legacy Computing](https://www.unicode.org/charts/PDF/U1FB00.pdf)
+   block from U+1FB00 through U+1FBAF, except U+1FB70–U+1FB7F.
 
 ### boxes
+
+The `boxes` capability indicates that box characters are drawn by the terminal
+itself, rather than through rendering the current font. This should cover at
+least:
+
+* The entirety of the [Box Drawing](https://www.unicode.org/charts/PDF/U2500.pdf)
+   block (U+2500–U+257F).
+* Symbols U+1FB70–U+1FB7F from the [Symbols for Legacy Computing](https://www.unicode.org/charts/PDF/U1FB00.pdf) block.