enum: remove arbitrary one-argument limitation

b_enum() contains a check that exactly one argument is given:

237:	if (error_info.errors || !*argv || *(argv + 1))

But the subsequent argument handling loop will happily deal with
multiple arguments:

246:	while(cp = *argv++)

Every other declaration command supports multiple arguments and I
see no reason why enum shouldn't. Simply removing the '*(argv + 1)'
check allows 'enum' to create more than one type per invocation.

src/cmd/ksh93/bltins/enum.c:
- b_enum(): Remove check for >1 args as described above.
- Update documentation to describe the behaviour of enumeration
  types in arithmetic expressions and to add an example: a bool
  type with two enumeration values 'false' (0) and 'true' (1).
  That type is predefined in ksh 93v- and 2020. We're not going
  to do that in 93u+m but it's good to document the possibility.

src/cmd/ksh93/sh.1:
- Make changes parallel to the enum.c self-doc update.

NEWS

@@ -3,6 +3,10 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-03-05:
+
+- The 'enum' command can now create more than one type per invocation.
+
 2022-02-23:
 
 - When reading input from the keyboard, ksh now turns off nonblocking I/O