Document --fast levels

**Is your feature request related to a problem? Please describe.**
Manpage says `--fast` can take levels starting at 1, but not which

**Describe the solution you'd like**
Document what is permitted and also what is reasonable

**Describe alternatives you've considered**
Leaving it as is

**Additional context**
https://github.com/facebook/zstd/blob/v1.4.8/lib/zstd.h#L108-L109
Internally looks like `1 << 17` for version I have installed