internal/export/unicode: change C comment to mention unassigned code points

As of CL 641395, C includes unassigned code points.
(It always has in the Unicode definition.)
Document that.

Change-Id: Icab0ef37e6efeebb1e2882f7743e29acc7ab77d8
Reviewed-on: https://go-review.googlesource.com/c/text/+/653615
Auto-Submit: Sean Liao <[email protected]>
Reviewed-by: Sean Liao <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc

Diff for: internal/export/unicode/gen.go

@@ -281,7 +281,7 @@ func printCategories() {
 		varDecl := ""
 		switch name {
 		case "C":
-			varDecl = "\tOther = _C;	// Other/C is the set of Unicode control and special characters, category C.\n"
+			varDecl = "\tOther = _C;	// Other/C is the set of Unicode control, special, and unassigned code points, category C.\n"
 			varDecl += "\tC = _C\n"
 		case "L":
 			varDecl = "\tLetter = _L;	// Letter/L is the set of Unicode letters, category L.\n"