Merge #16149: doc: Rework section on ACK in CONTRIBUTING.md

fac5ddfc57 doc: Rework section on ACK (MarcoFalke)

Pull request description:

  `utACK` and `t(ested) ACK` are deprecated and will be removed in the next major release. Please use the more generic `ACK` and include an explanation of what was reviewed.

  There was a related discussion in http://diyhpl.us/wiki/transcripts/bitcoin-core-dev-tech/2019-06-05-code-review/ section `The author could offer a guide for review`.

ACKs for commit fac5dd:
  moneyball:
    ACK fac5ddfc5796022601af2c17fd0b158b2c465cba

Tree-SHA512: 29177e8d96aeba055b5cad6d99be3ca1be0c61af0fdc90f70a3136872c9ad6201a02f63fbac78b90b8a56b4c06af304f2583d52a94fdd954fdcc7ad0552b9ef8

CONTRIBUTING.md

@@ -210,7 +210,22 @@ request. Typically reviewers will review the code for obvious errors, as well as
 test out the patch set and opine on the technical merits of the patch. Project
 maintainers take into account the peer review when determining if there is
 consensus to merge a pull request (remember that discussions may have been
-spread out over GitHub, mailing list and IRC discussions). The following
+spread out over GitHub, mailing list and IRC discussions).
+
+#### Conceptual Review
+
+A review can be a conceptual review, where the reviewer leaves a comment
+ * `Concept (N)ACK`, meaning "I do (not) agree in the general goal of this pull
+   request",
+ * `Approach (N)ACK`, meaning `Concept ACK`, but "I do (not) agree with the
+   approach of this change".
+
+#### Code Review
+
+After conceptual agreement on the change, code review can be provided. It is
+starting with `ACK BRANCH_COMMIT`, where `BRANCH_COMMIT` is the top of the
+topic branch. The review is followed by a description of how the reviewer did
+the review. The following
 language is used within pull-request comments:
 
   - (t)ACK means "I have tested the code and I agree it should be merged", involving
@@ -222,12 +237,8 @@ language is used within pull-request comments:
     issues, legal justification). NACKs without accompanying reasoning may be
     disregarded;
   - utACK means "I have not tested the code, but I have reviewed it and it looks
-    OK, I agree it can be merged";
-  - Concept ACK means "I agree in the general principle of this pull request";
   - Nit refers to trivial, often non-blocking issues.
 
-Reviewers should include the commit hash which they reviewed in their comments.
-
 Project maintainers reserve the right to weigh the opinions of peer reviewers
 using common sense judgement and also may weight based on meritocracy: Those
 that have demonstrated a deeper commitment and understanding towards the project