Merge #20830: doc: update developer notes with signet

ee701a9 doc: update developer notes for signet (Jon Atack)

Pull request description:

  Preview with working anchor links:

  https://github.com/jonatack/bitcoin/blob/add-signet-to-developer-notes/doc/developer-notes.md

ACKs for top commit:
  0xB10C:
    ACK ee701a9
  michaelfolkson:
    ACK ee701a9
  MarcoFalke:
    ACK ee701a9

Tree-SHA512: 12f0a81a701d9fd48e1f314e7a66b865309fdcb8f139323410d35c0cfb093d23a2f899249d1b0a025078894cf4789ed7221ae42804a56ab704a8b9ad35077bac

Author: MarcoFalke

doc/developer-notes.md

@@ -14,7 +14,7 @@ Developer Notes
         - [Compiling for debugging](#compiling-for-debugging)
         - [Compiling for gprof profiling](#compiling-for-gprof-profiling)
         - [`debug.log`](#debuglog)
-        - [Testnet and Regtest modes](#testnet-and-regtest-modes)
+        - [Signet, testnet, and regtest modes](#signet-testnet-and-regtest-modes)
         - [DEBUG_LOCKORDER](#debug_lockorder)
         - [Valgrind suppressions file](#valgrind-suppressions-file)
         - [Compiling for test coverage](#compiling-for-test-coverage)
@@ -259,14 +259,15 @@ on all categories (and give you a very large `debug.log` file).
 The Qt code routes `qDebug()` output to `debug.log` under category "qt": run with `-debug=qt`
 to see it.
 
-### Testnet and Regtest modes
+### Signet, testnet, and regtest modes
 
-Run with the `-testnet` option to run with "play bitcoins" on the test network, if you
-are testing multi-machine code that needs to operate across the internet.
+If you are testing multi-machine code that needs to operate across the internet,
+you can run with either the `-signet` or the `-testnet` config option to test
+with "play bitcoins" on a test network.
 
-If you are testing something that can run on one machine, run with the `-regtest` option.
-In regression test mode, blocks can be created on-demand; see [test/functional/](/test/functional) for tests
-that run in `-regtest` mode.
+If you are testing something that can run on one machine, run with the
+`-regtest` option.  In regression test mode, blocks can be created on demand;
+see [test/functional/](/test/functional) for tests that run in `-regtest` mode.
 
 ### DEBUG_LOCKORDER