doc: list the I2P accept thread in developer-notes

Author: vasild

doc/developer-notes.md

@@ -646,6 +646,9 @@ Threads
   - [ThreadOpenConnections (`b-opencon`)](https://doxygen.bitcoincore.org/class_c_connman.html#a55e9feafc3bab78e5c9d408c207faa45)
     : Initiates new connections to peers.
 
+  - [ThreadI2PAcceptIncoming (`b-i2paccept`)](https://doxygen.bitcoincore.org/class_c_connman.html#a57787b4f9ac847d24065fbb0dd6e70f8)
+    : Listens for and accepts incoming I2P connections through the I2P SAM proxy.
+
 Ignoring IDE/editor files
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