doc: describe 'init load' thread actions

Author: mzumsande

doc/developer-notes.md

@@ -621,8 +621,9 @@ Threads
   : Started from `main()` in `bitcoind.cpp`. Responsible for starting up and
   shutting down the application.
 
-- [ThreadImport (`b-initload`)](https://doxygen.bitcoincore.org/namespacenode.html#ab4305679079866f0f420f7dbf278381d)
-  : Loads blocks from `blk*.dat` files or `-loadblock=<file>` on startup.
+- [Init load (`b-initload`)](https://doxygen.bitcoincore.org/namespacenode.html#ab4305679079866f0f420f7dbf278381d)
+  : Performs various loading tasks that are part of init but shouldn't block the node from being started: external block import,
+   reindex, reindex-chainstate, main chain activation, spawn indexes background sync threads and mempool load.
 
 - [CCheckQueue::Loop (`b-scriptch.x`)](https://doxygen.bitcoincore.org/class_c_check_queue.html#a6e7fa51d3a25e7cb65446d4b50e6a987)
   : Parallel script validation threads for transactions in blocks.