Add Rego v1 keywords (open-policy-agent#6649)

Let me know if these should be in a box of their own — my reasoning
was that they should be avoided even prior to that, so if people won't
use them as var names or whatever, all the better.

Signed-off-by: Anders Eknert <anders@eknert.com>
Signed-off-by: Thomas Sidebottom <thomas.sidebottom@va.gov>

docs/content/policy-reference.md

@@ -1316,10 +1316,16 @@ names, or dot-access style reference arguments:
 
 ```
 as
+contains
+data
 default
 else
+every
 false
+if
+in
 import
+input
 package
 not
 null
@@ -1381,7 +1387,7 @@ non-empty-set   = "{" term { "," term } "}"
 empty-set       = "set(" ")"
 ```
 
-Note that the grammar corresponds to Rego with all future keywords enabled.
+Note that the grammar corresponds to Rego with `rego.v1` enabled.
 
 The grammar defined above makes use of the following syntax. See [the Wikipedia page on EBNF](https://en.wikipedia.org/wiki/Extended_Backus–Naur_Form) for more details: