doc: expand description of font units

stylix/fonts.nix

@@ -61,41 +61,55 @@ in
       };
     };
 
-    sizes = {
-      desktop = lib.mkOption {
-        description = ''
-          The font size (in pt) used in window titles/bars/widgets elements of
-          the desktop.
-        '';
-        type = with lib.types; (either ints.unsigned float);
-        default = 10;
-      };
+    sizes =
+      let
+        mkFontSizeOption =
+          { default, target }:
+          lib.mkOption {
+            inherit default;
 
-      applications = lib.mkOption {
-        description = ''
-          The font size (in pt) used by applications.
-        '';
-        type = with lib.types; (either ints.unsigned float);
-        default = 12;
-      };
+            description = ''
+              The font size used for ${target}.
 
-      terminal = lib.mkOption {
-        description = ''
-          The font size (in pt) for terminals/text editors.
-        '';
-        type = with lib.types; (either ints.unsigned float);
-        default = cfg.sizes.applications;
-      };
+              This is measured in [points](https://en.wikipedia.org/wiki/Point_(typography)).
+              In a computing context, there should be 72 points per inch.
+
+              [The CSS specification](https://drafts.csswg.org/css-values/#absolute-lengths)
+              says there should be 96 reference pixels per inch. This means CSS
+              uses a fixed ratio of 3 points to every 4 pixels, which is
+              sometimes useful. However, reference pixels might not correspond
+              to physical pixels, so this conversion may be invalid for other
+              applications.
+
+              The measurements given in inches are likely to be incorrect
+              unless you've
+              [manually set your DPI](https://linuxreviews.org/HOWTO_set_DPI_in_Xorg).
+            '';
 
-      popups = lib.mkOption {
-        description = ''
-          The font size (in pt) for notifications/popups and in general overlay
-          elements of the desktop.
-        '';
-        type = with lib.types; (either ints.unsigned float);
-        default = cfg.sizes.desktop;
+            type = with lib.types; either ints.unsigned float;
+          };
+      in
+      {
+        desktop = mkFontSizeOption {
+          target = "window titles, status bars, and other general elements of the desktop";
+          default = 10;
+        };
+
+        applications = mkFontSizeOption {
+          target = "applications";
+          default = 12;
+        };
+
+        terminal = mkFontSizeOption {
+          target = "terminals and text editors";
+          default = cfg.sizes.applications;
+        };
+
+        popups = mkFontSizeOption {
+          target = "notifications, popups, and other overlay elements of the desktop";
+          default = cfg.sizes.desktop;
+        };
       };
-    };
 
     packages = lib.mkOption {
       description = ''