Document the preference of nullptr over NULL or (void*)0

doc/developer-notes.md

@@ -37,6 +37,7 @@ code.
 
 - **Miscellaneous**
   - `++i` is preferred over `i++`.
+  - `nullptr` is preferred over `NULL` or `(void*)0`.
   - `static_assert` is preferred over `assert` where possible. Generally; compile-time checking is preferred over run-time checking.
 
 Block style example:
@@ -276,7 +277,7 @@ Wallet
 
   - *Rationale*: In RPC code that conditionally uses the wallet (such as
     `validateaddress`) it is easy to forget that global pointer `pwalletMain`
-    can be NULL. See `test/functional/disablewallet.py` for functional tests
+    can be nullptr. See `test/functional/disablewallet.py` for functional tests
     exercising the API with `-disablewallet`
 
 - Include `db_cxx.h` (BerkeleyDB header) only when `ENABLE_WALLET` is set