-The function `unibyte-char-to-multibyte' is used to convert
-each unibyte character to a multibyte character. */)
+If STRING is unibyte and contains non-ASCII characters, the function
+`unibyte-char-to-multibyte' is used to convert each unibyte character
+to a multibyte character. In this case, the returned string is a
+newly created string with no text properties. If STRING is multibyte
+or entirely ASCII, it is returned unchanged. In particular, when
+STRING is unibyte and entirely ASCII, the returned string is unibyte.
+\(When the characters are all ASCII, Emacs primitives will treat the
+string the same way whether it is unibyte or multibyte.) */)