doc: add rpc guidance for boolean verbosity avoidance

Author: tdb3

doc/developer-notes.md

@@ -1397,6 +1397,12 @@ A few guidelines for introducing and reviewing new RPC interfaces:
     to a multi-value, or due to other historical reasons. **Always** have false map to 0 and
     true to 1 in this case.
 
+- For new RPC methods, if implementing a `verbosity` argument, use integer verbosity rather than boolean.
+  Disallow usage of boolean verbosity (see `ParseVerbosity()` in [util.h](/src/rpc/util.h)).
+
+  - *Rationale*: Integer verbosity allows for multiple values. Undocumented boolean verbosity is deprecated
+    and new RPC methods should prevent its use.
+
 - Don't forget to fill in the argument names correctly in the RPC command table.
 
   - *Rationale*: If not, the call cannot be used with name-based arguments.