Add release note snippit for CPPFLAGS handing [skip ci]

docs/markdown/snippets/preproc-flags.md

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+## (C) Preprocessor flag handling
+
+Meson previously stored `CPPFLAGS` and per-language compilation flags
+separately. (That latter would come from `CFLAGS`, `CXXFLAGS`, etc., along with
+`<lang>_args` options whether specified no the command-line interface (`-D..`),
+`meson.build` (`default_options`), or cross file (`[properties]`).) This was
+mostly unobservable, except for certain preprocessor-only checks like
+`check_header` would only use the preprocessor flags, leading to confusion if
+some `-isystem` was in `CFLAGS` but not `CPPFLAGS`. Now, they are lumped
+together, and `CPPFLAGS`, for the languages which are deemed to care to about,
+is just another source of compilation flags along with the others already
+listed.