Say in the manual and the register that colour labels exist

The manual's library section described rating and flagging only, and
the outstanding register still said colour labels were "set and shown
nowhere" and that three NFR-A11Y-3 clauses had no test. Both are now
untrue: the manual gives the keys, the Label button and the chips, and
the register names the test file and what it can and cannot vouch for.

docs/manual/README.md

@@ -33,8 +33,8 @@ wrote first.
 
 Photographs are ordered by capture time, with a month heading where each
 begins. The strip on the left is the timeline — drag it to jump to a year.
-The bar above the grid filters by rating, flag and where the file is (on this
-device, or only on the server).
+The bar above the grid filters by rating, flag, colour label and where the
+file is (on this device, or only on the server).
 
 ### Rating and flagging
 
@@ -43,6 +43,14 @@ count what each rating holds, and clicking `3+` shows only those.
 
 ![Rating two photographs, then filtering the grid to three stars and more](media/library-rating.gif)
 
+Colour labels work as Lightroom's do: `6` red, `7` yellow, `8` green, `9`
+blue, on the photograph under the pointer or on the selection, and the same
+key again takes the label off. `Label` on the selection bar offers all five,
+purple included, and `None`. Each label is drawn with its initial on it, so
+it reads without telling the colours apart, and the filter bar has a chip for
+each. In develop, the top bar names the open photograph's label and sets it,
+and the same keys work there.
+
 ### Getting about
 
 Drag the timeline to scrub through years; Ctrl and the wheel resize the