[3.8] bpo-37253: Document PyCompilerFlags.cf_feature_version (GH-14019) (GH-14038)

* Update PyCompilerFlags structure documentation.
* Document the new cf_feature_version field in the Changes in the C
  API section of the What's New in Python 3.8 doc.

(cherry picked from commit 2c9b498759f4fc74da82a0a96d059d666fa73f16)
2 files changed