Merge bitcoin/bitcoin#30142: doc: add guidance for RPC to developer notes

c6eca6f doc: add guidance for RPC to developer notes (tdb3)

Pull request description:

  Adds guidance statements to the RPC interface section of the developer notes with examples of when to implement `-deprecatedrpc=`.

  Wanted to increase awareness of preferred RPC implementation approaches for newer contributors.

  This implements some of what's discussed in bitcoin/bitcoin#29912 (comment)

  Opinions may differ, so please don't be shy.  We want to make RPC as solid/safe as possible.

  Examples of discussions where guidelines/context might have added value:
  bitcoin/bitcoin#30212 (comment)
  bitcoin/bitcoin#29845 (comment)
  bitcoin/bitcoin#30381 (review)
  bitcoin/bitcoin#29954 (comment)
  bitcoin/bitcoin#30410 (review)
  bitcoin/bitcoin#30713
  bitcoin/bitcoin#30381
  bitcoin/bitcoin#29060 (review)

ACKs for top commit:
  l0rinc:
    ACK c6eca6f
  fjahr:
    ACK c6eca6f
  maflcko:
    lgtm ACK c6eca6f
  jonatack:
    ACK c6eca6f

Tree-SHA512: 01a98a8dc0eb91762b225d3278cdb4a5e380ceb7486fd096b4ad9122bed859cea8584d8996d3dce51272fdb792f4a793a1bd1c5445efeb87f0a30f9b6e59a790

Author: fanquake

doc/developer-notes.md

@@ -1468,6 +1468,12 @@ A few guidelines for introducing and reviewing new RPC interfaces:
   - *Rationale*: JSON strings are Unicode strings, not byte strings, and
     RFC8259 requires JSON to be encoded as UTF-8.
 
+A few guidelines for modifying existing RPC interfaces:
+
+- It's preferable to avoid changing an RPC in a backward-incompatible manner, but in that case, add an associated `-deprecatedrpc=` option to retain previous RPC behavior during the deprecation period. Backward-incompatible changes include: data type changes (e.g. from `{"warnings":""}` to `{"warnings":[]}`, changing a value from a string to a number, etc.), logical meaning changes of a value, or key name changes (e.g. `{"warning":""}` to `{"warnings":""}`). Adding a key to an object is generally considered backward-compatible. Include a release note that refers the user to the RPC help for details of feature deprecation and re-enabling previous behavior. [Example RPC help](https://github.com/bitcoin/bitcoin/blob/94f0adcc/src/rpc/blockchain.cpp#L1316-L1323).
+
+  - *Rationale*: Changes in RPC JSON structure can break downstream application compatibility. Implementation of `deprecatedrpc` provides a grace period for downstream applications to migrate. Release notes provide notification to downstream users.
+
 Internal interface guidelines
 -----------------------------