doc: clarify that LOCK() does AssertLockNotHeld() internally

Constructs like

```cpp
AssertLockNotHeld(m);
LOCK(m);
```

are equivalent to

```cpp
LOCK(m);
```

for non-recursive mutexes, so it is ok to omit `AssertLockNotHeld()` in
such cases.

doc/developer-notes.md

@@ -941,7 +941,9 @@ Threads and synchronization
     internal to a class (private or protected) rather than public.
 
   - Combine annotations in function declarations with run-time asserts in
-    function definitions:
+    function definitions (`AssertLockNotHeld()` can be omitted if `LOCK()` is
+    called unconditionally after it because `LOCK()` does the same check as
+    `AssertLockNotHeld()` internally, for non-recursive mutexes):
 
 ```C++
 // txmempool.h