bug#35710: 26.1; incomplete mapconcat description string

[phs <phs@lsv.fr>]

In my v26.1, the description string for mapconcat is

   mapconcat is a built-in function in ‘src/fns.c’.

   (mapconcat FUNCTION SEQUENCE SEPARATOR)

   Apply FUNCTION to each element of SEQUENCE, and concat the results as
strings.
   In between each pair of results, stick in SEPARATOR.  Thus, " " as
   SEPARATOR results in spaces between the values returned by FUNCTION.
   SEQUENCE may be a list, a vector, a bool-vector, or a string.

The description does not say what is accepted for SEPARATOR, it only
gives an example.

By playing around a bit, it turns out that SEPARATOR can be a string
(obviously), a list of chars, a vector of chars, the empty bool-vector
but not any other bool-vector.

It is not clear why these possibilities are offered while some are not
(e.g., why not make SEPARATOR optional?). This is one more reason to
make the description string explicit about what is allowed.
-- 
Philippe SCHNOEBELEN,       	
"Algebraic symbols are used when you do not know what you are talking about"