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.

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2022-02-23"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2022-03-05"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */