Docs: git pull is needed after syncing git modules (project-chip#29299)

* git pull is needed after syncing git modules

I followed the instructions to update the repo but still wondered why it dit not really do anything. Now I stumbled the secod time over the fact that tthe "clone" command in docs give you a full working repo, but the "synching modules" does not update them ... so git pull is needed.

I think it nmight be good to add that here in order to support users.

* Update BUILDING.md

* Update docs/guides/BUILDING.md

Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>

* Restyled by prettier-markdown

---------

Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>
Co-authored-by: Restyled.io <commits@restyled.io>

docs/guides/BUILDING.md

@@ -31,12 +31,13 @@ To check out the Matter repository, run the following command:
 git clone --recurse-submodules git@github.com:project-chip/connectedhomeip.git
 ```
 
-## Synchronizing submodules
+## Updating Matter code
 
-If you already have the Matter code checked out, run the following command to
-synchronize submodules:
+If you already have the Matter code checked out, run the following commands to
+update the repository and synchronize submodules:
 
 ```
+git pull
 git submodule update --init
 ```