bug#1727: 23.0.60; char syntax for non-breaking space

[Noam Postavsky]

The discrepancy between char and string escaping syntax is currently
documented under "Non-ASCII Characters in Strings":

   You can also use hexadecimal escape sequences (‘\xN’) and octal
escape sequences (‘\N’) in string constants.  *But beware:* If a string
constant contains hexadecimal or octal escape sequences, and these
escape sequences all specify unibyte characters (i.e., less than 256),
and there are no other literal non-ASCII characters or Unicode-style
escape sequences in the string, then Emacs automatically assumes that it
is a unibyte string.  That is to say, it assumes that all non-ASCII
characters occurring in the string are 8-bit raw bytes.