docs: document foreground_text_hsb

It even has screen shots!

refs: #491

docs/config/lua/config/foreground_text_hsb.md

@@ -0,0 +1,28 @@
+# foreground_text_hsb
+
+*Since: nightly builds only*
+
+Configures a Hue, Saturation, Brightness transformation that is applied to
+monochrome glyphs.
+
+The transform works by converting the RGB colors to HSV values and
+then multiplying the HSV by the numbers specified in `foreground_text_hsb`.
+
+Modifying the hue changes the hue of the color by rotating it through the color
+wheel. It is not as useful as the other components, but is available "for free"
+as part of the colorspace conversion.
+
+Modifying the saturation can add or reduce the amount of "colorfulness". Making
+the value smaller can make it appear more washed out.
+
+Modifying the brightness can be used to dim or increase the perceived amount of
+light.
+
+The range of these values is 0.0 and up; they are used to multiply the existing
+values, so the default of 1.0 preserves the existing component, whilst 0.5 will
+reduce it by half, and 2.0 will double the value.
+
+<img src="../../../screenshots/foreground-text-hsb-1-1-1.png">
+<img src="../../../screenshots/foreground-text-hsb-1-1.5-1.png">
+<img src="../../../screenshots/foreground-text-hsb-1-1-1.5.png">
+<img src="../../../screenshots/foreground-text-hsb-1.5-1-1.png">