patch 8.2.0901: formatting CJK text isn't optimal

Problem:    Formatting CJK text isn't optimal.
Solution:   Properly break CJK lines. (closes #3875)

runtime/doc/change.txt

@@ -1688,6 +1688,10 @@ B	When joining lines, don't insert a space between two multi-byte
 	characters.  Overruled by the 'M' flag.
 1	Don't break a line after a one-letter word.  It's broken before it
 	instead (if possible).
+]	Respect textwidth rigorously. With this flag set, no line can be
+	longer than textwidth, unless line-break-prohibition rules make this
+	impossible.  Mainly for CJK scripts and works only if 'encoding' is
+	"utf-8".
 j	Where it makes sense, remove a comment leader when joining lines.  For
 	example, joining:
 		int i;   // the index ~