Merge bitcoin/bitcoin#31819: doc: swap CPPFLAGS for APPEND_CPPFLAGS

ea687d2 doc: swap CPPFLAGS for APPEND_CPPFLAGS (fanquake)

Pull request description:

  `APPEND_CPPFLAGS` will be understood by our CMake, whereas `CPPFLAGS` will not. Attempting what is currently documented will just give:
  ```bash
  CMake Warning:
    Ignoring extra path from command line:

     "CPPFLAGS=-DDEBUG_LOCKCONTENTION"
  ```

ACKs for top commit:
  fjahr:
    ACK ea687d2
  hebasto:
    ACK ea687d2.

Tree-SHA512: b8d3359b77a813535a4fa715619b815cd88e5440950f7d4cd045514e6b45d3f1c7f62061315c8581d0a99c0aec38340d34008be05657d198d868b241d19b7828

doc/developer-notes.md

@@ -432,8 +432,8 @@ RPC that, when enabled, logs the location and duration of each lock contention
 to the `debug.log` file.
 
 The `-DCMAKE_BUILD_TYPE=Debug` build option adds `-DDEBUG_LOCKCONTENTION` to the
-compiler flags. You may also enable it manually by building with `-DDEBUG_LOCKCONTENTION` added to your CPPFLAGS,
-i.e. `CPPFLAGS="-DDEBUG_LOCKCONTENTION"`, then build and run bitcoind.
+compiler flags. You may also enable it manually by building with `-DDEBUG_LOCKCONTENTION`
+added to your CPPFLAGS, i.e. `-DAPPEND_CPPFLAGS="-DDEBUG_LOCKCONTENTION"`.
 
 You can then use the `-debug=lock` configuration option at bitcoind startup or
 `bitcoin-cli logging '["lock"]'` at runtime to turn on lock contention logging.