Make outputHashAlgo accept "nar", stay in sync

Now that we have a few things identifying content address methods by
name, we should be consistent about it.

Move up the `parseHashAlgoOpt` for tidiness too.

Discussed this change for consistency's sake as part of #8876

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

doc/manual/src/language/advanced-attributes.md

@@ -207,12 +207,17 @@ Derivations can declare some infrequently used optional attributes.
 
         This is the default.
 
-      - `"recursive"`\
-        The hash is computed over the NAR archive dump of the output
+      - `"recursive"` or `"nar"`\
+        The hash is computed over the [NAR archive](@docroot@/glossary.md#gloss-nar) dump of the output
         (i.e., the result of [`nix-store --dump`](@docroot@/command-ref/nix-store/dump.md)). In
         this case, the output can be anything, including a directory
         tree.
 
+    `"recursive"` is the traditional way of indicating this,
+    and is supported since 2005 (virtually the entire history of Nix).
+    `"nar"` is more clear, and consistent with other parts of Nix (such as the CLI),
+    however support for it is only added in Nix version 2.21.
+
   - [`__contentAddressed`]{#adv-attr-__contentAddressed}
     > **Warning**
     > This attribute is part of an [experimental feature](@docroot@/contributing/experimental-features.md).