bug#28774: Master, emacs-26: Can't add text property to built-in function name.

[Drew Adams]

> It seems ido-completions relies on (format "%s" str) to return a copy of
> str.  This fixes it:

If `format' no longer always copies STR in this context then
its doc should be updated, I think.

This:

 Format a string out of a format-string and arguments.
 The first argument is a format control string.
 The other arguments are substituted into it to make the
 result, a string.

suggests that it creates a new string, or at most reuses
the format string (e.g., "%s"), modifying it by substituting
STR for %s in it.  If in fact it can sometimes simply return
STR then this should be mentioned explicitly, to avoid confusion.

(And why was this change made?  Was it just to save a string
copy?)

This is an incompatible Lisp change, if it is new.  In that
case, it should be documented as such.