- "Convert SPEC (in inches, millimeters, points, or picas) into points."
- ;; 1 in = 6 pa = 25.4 mm = 72 pt
+ "Convert SPEC (in inches, millimeters, points, picas, or pixels) into points.
+
+Canonical sizes are in points. If SPEC is null, nil is returned. If SPEC is
+a number, it is interpreted as the desired point size and returned unchanged.
+Otherwise SPEC must be a string consisting of a number and an optional type.
+The type may be the strings \"px\", \"pix\", or \"pixel\" (pixels), \"pt\" or
+\"point\" (points), \"pa\" or \"pica\" (picas), \"in\" or \"inch\" (inches),
+\"cm\" (centimeters), or \"mm\" (millimeters).
+
+1 in = 2.54 cm = 6 pa = 25.4 mm = 72 pt. Pixel size is device-dependent."