Revert "Allow any unicode characters in the TagValue #71" (#89)

Author: songy23

tags/TagContext.md

@@ -20,8 +20,8 @@ A string or string wrapper, with some restrictions:
 
 ## TagValue
 
-A TagValue can be any unicode string whose size when encoded in UTF-8
-is less than 256 bytes. An empty value is allowed.
+A string or string wrapper with the same restrictions as `TagKey`, except that it
+is allowed to be empty.
 
 ## Serialization