Clarify isblank range

lib/libc/locale/isblank.3

@@ -48,6 +48,15 @@
 The
 .Fn isblank
 function tests for a space or tab character.
+For any locale, this includes the following standard characters:
+.Pp
+.Bl -column \&`\et''___ \&``\et''___
+.It "\&``\et''\t`` ''"
+.El
+.Pp
+In the "C" locale
+.Fn isblank
+successful test is limited to this characters only.
 For single C
 .Va char Ns s
 locales (see
@@ -66,3 +75,8 @@ returns non-zero if the character tests true.
 .Xr ctype 3 ,
 .Xr multibyte 3 ,
 .Xr ascii 7
+.Sh STANDARDS
+The
+.Fn isblank
+function conforms to
+.St -p1003.1 .