Document byte-to-string in Lisp manual.

Document byte-to-string in Lisp manual.

* objects.texi (Symbol Type):
* text.texi (Near Point):
* help.texi (Help Functions):
* functions.texi (Mapping Functions): Use string instead of
char-to-string in examples.

* nonascii.texi (Converting Representations): Document
byte-to-string.

* strings.texi (Creating Strings): Don't mention semi-obsolete
function char-to-string.
(String Conversion): Shorten discussion of semi-obsolete function
string-to-char.  Link to Converting Representations.