Document pass-through behavior in zstd.1 and --help

[cgbur]

Document pass-through:

• --help
• man

Document the always-on pass-through behavior when the -f flag is present and when writing to stdout during decompression:

• --help
• man

Open question: should the current pass-through be documented in the -f flag or is it too nuanced to go there. Perhaps all of the information should be in the pass-through flag documentation?

I will have time this weekend to open a pull request if there's no objection.

also as a note to whoever fixes this theres a misplaced \n in the help string for pass-through