import: crate: Generate comments with ‘TODO REVIEW:’ prefix.

This distinguishes them from manually added ones.  When committing changes,
these ‘TODO REVIEW’ comments should be addressed and removed first.

If you're working with Rust packaging, please update your local Guix to this
commit to apply the change.

* guix/import/crate.scm (cargo-lock->expressions): Generate comments with
‘TODO REVIEW:’ prefix.
* tests/import/crate.scm ("crate-lockfile-import"): Don't check the exact
comment string.
* doc/guix-cookbook.texi (Common Workflow for Rust Packaging),
* gnu/packages/rust-crates.scm: Adjust accordingly.

Change-Id: I76b914b49be46d04cc61b101b6bc9e31cadb7f07
Signed-off-by: Rutherther <rutherther@ditigal.xyz>
Merges: #4567

tests/import/crate.scm

@@ -311,9 +311,7 @@ version = \"25.2.0\"\n" port)))
                 "aho-corasick" "1.1.3"
                 "05mrpkvdgp5d20y2p989f187ry9diliijgwrs254fs9s1m1x6q4f"))
              (define rust-smithay-0.4.0.0cd3345
-               ,($ <comment>
-                   ";; TODO: Define standalone package if this is a workspace.\n"
-                   #f)
+               ,(? comment?)
                (origin
                  (method git-fetch)
                  (uri (git-reference