[clang][python] Rename ClangObject to OpaqueClangObject and update its description (#229374)
This patch documents the existing way `ClangObject` is used in
`cindex.py` and renames it to `OpaqueClangObject` to better reflect its
purpose. In one case (`CodeCompletionResults`) it's used as a base class
for objects that wrap transparent C objects, but this case will be
eliminated by #224678.
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Co-authored-by: Jannick Kremer <jannick.kremer@mailbox.org>