docs(sync): Note repos not found behavior

Author: tony

docs/cli/sync.md

@@ -6,9 +6,35 @@
 
 ## Error handling
 
+### Repos not found in config
+
+As of 1.13.x, if you enter a repo term (or terms) that aren't found throughout
+your configurations, it will show a warning:
+
+```console
+$ vcspull sync non_existent_repo
+No repo found in config(s) for "non_existent_repo"
+```
+
+```console
+$ vcspull sync non_existent_repo existing_repo
+No repo found in config(s) for "non_existent_repo"
+```
+
+```console
+$ vcspull sync non_existent_repo existing_repo another_repo_not_in_config
+No repo found in config(s) for "non_existent_repo"
+No repo found in config(s) for "another_repo_not_in_config"
+```
+
+Since syncing terms are treated as a filter rather than a lookup, the message is
+considered a warning, so will not exit even if `--exit-on-error` flag is used.
+
+### Syncing
+
 As of 1.13.x, vcspull will continue to the next repo if an error is encountered when syncing multiple repos.
 
-To imitate the old behavior, use `--exit-on-error` / `-x`:
+To imitate the old behavior, the `--exit-on-error` / `-x` flag:
 
 ```console
 $ vcspull sync --exit-on-error grako django