Merge pull request libgit2#6109 from joshtriplett/document-odb-race-free

Document that `git_odb` is thread-safe

Author: ethomson

docs/threading.md

@@ -21,6 +21,9 @@ There are some objects which are read-only/immutable and are thus safe
 to share across threads, such as references and configuration
 snapshots.
 
+The `git_odb` object uses locking internally, and is thread-safe to use from
+multiple threads simultaneously.
+
 Error messages
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