Doc clarifications on xargs non-use of the shell, POSIX compliance of several options and release years of findutils versions

This small sequence of documentation changes includes:

1. Clarify when xargs options -l, -i, -e were removed from POSIX.
2. Clarify when xargs options -p, and -r were added to POSIX.
3. Clarify that xargs does not use the shell to launch commands.
4. When mentioning findutils releases, indicate the year of the release.