clarify unit modifiers for ` --format` in the manpage

# Bug Report

I did not understand what the unit suffixes for `--format` meant from the man page.

# Enhancement Request

* Desired behavior

clarify the manpage.

[Instead of the form](https://github.com/esnet/iperf/blob/master/src/iperf3.1#L113)
`.BR -f ", " --format " "
[kmgtKMGT] format to report: Kbits/Mbits/Gbits/Tbits`

write
`.BR -f ", " --format " "
[kmgtKMGT] format to report: kmgt bit/s or KMGT byte/s`

[Source](https://github.com/esnet/iperf/blob/master/src/units.c#L257)
```
* Given a number in bytes and a format, converts the number and
* prints it out with a bits or bytes label.
* B, K, M, G, A for Byte, Kbyte, Mbyte, Gbyte, adaptive byte
* b, k, m, g, a for bit, Kbit, Mbit, Gbit, adaptive bit
```