Merge bitcoin/bitcoin#33568: doc: how to update a subtree

a1226bc doc: how to update a subtree (Sjors Provoost)

Pull request description:

  We have instructions on how to verify a subtree update, but not on how to perform one.

ACKs for top commit:
  yuvicc:
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  achow101:
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  janb84:
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  furszy:
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Tree-SHA512: ba3ccc56a9f1c7f461e0db9699612e1fd64b7c72bfd1dae63d4cb830db416871a493820d3a7924c19b6ce353fc20c5fe07578b053dec6ea68273a007cbebc512

Author: achow101

doc/developer-notes.md

@@ -1119,6 +1119,19 @@ to check a subtree directory for consistency with its upstream repository.
 
 The tool instructions also include a list of the subtrees managed by Bitcoin Core.
 
+To fully verify or update a subtree, add it as a remote:
+
+```sh
+git remote add libmultiprocess https://github.com/bitcoin-core/libmultiprocess.git
+```
+
+To update the subtree:
+
+```sh
+git fetch libmultiprocess
+git subtree pull --prefix=src/ipc/libmultiprocess libmultiprocess master --squash
+```
+
 The ultimate upstream of the few externally managed subtrees are:
 
 - src/leveldb