Document that zstd:chunked doesn’t make sense with encryption

[mtrmac]

Encrypted images don’t benefit from zstd:chunked , and if asked to do so, the code uses non-chunked zstd (ref. https://github.com/containers/image/issues/2485 .)

Around https://github.com/containers/common/blob/1a8d1919e2cea08126f8e26c33562ffe2933086c/pkg/config/containers.conf#L442 , and for add_compression option: the documentation in comments and/or man pages [wherever the authoritative source is] should probably note that zstd:chunked is not used for encrypted images [and triggers warnings].