documentation: make more clear that payloads may be modified

[jowagner]

Pixz modifies the payload when it thinks that the input is a tar file. Not all users expect this from the 'i' or 'indexed' in the name. Small, careful changes to the command description and man page could reduce the surprises and possible data loss.

Consider also the possibility that pixz detects a tar file when the input is actually not a tar file, e.g. random data (low likelihood of false positives) or a malicious file (designed to be a false positive).