Docs: Document CLI command exit code

A community user recently encountered an issue where he/she tried to start a single-node-instance, but got exit code 6 without any log whatsoever. I learned from @knz that exit code 6 indicates a logging file unavailable error.
```go
// LoggingFileUnavailable (6) indicates that an error occurred
// during a logging operation to a file.
func LoggingFileUnavailable() Code { return Code{6} }
```

The desire/ask here is we should document [all return codes](https://github.com/cockroachdb/cockroach/blob/54282ad5728ded88c7b40c1c663c7e57ff6ac2b9/pkg/cli/exit/codes.go) so it's easier to understand what they mean.