Clarify versioning pragmas

- Add more explanation for the whole "perl 5 errors" thing, as it's
   unclear unless you already know what it's about
- Explain more about .PREVIEW version, though its mechanics are
    slightly fuzzy around the edges: R#2464 rakudo/rakudo#2464

doc/Language/pragmas.pod6

@@ -22,14 +22,18 @@ are to be defined later.)
 =item X<C<v6.x>|v6.x> This pragma states the version of the compiler that is going to be used,
 and turns on its features if they are optional.
 
-    use v6;   # Tells the Perl 5 interpreter this is not for it.
-    use v6.c; # Uses the "Christmas" version of Perl 6.
-    use v6.d.PREVIEW; # Turns on available "Diwali" features in v6.c
-    use v6.d; # Use "Diwali" features.
+    use v6;   # Load latest supported version (non-PREVIEW). 
+              # Also, useful for producing better errors when accidentally
+              # executing the program with `perl` instead of `perl6`
+              
+    use v6.c;         # Use the "Christmas" version of Perl 6
+    use v6.d;         # Use the "Diwali" version of Perl 6
+    use v6.d.PREVIEW; # On 6.d-capable compilers, enabled 6.s features,
+                      # otherwise enables the available experimental
+                      # preview features for 6.d language
 
 Since these pragmas turn on the compiler version, they should be the
-first line in the file.
-
+first statement in the file (preceeding comments and Pod are fine).
 
 =item X<B<MONKEY-GUTS>|MONKEY-GUTS>