doc: document optional suffix for --pad argument in flux-ping(1)

Problem: The flux-ping(1) --pad argument is now processed by
optparse_get_size_int(), which means it may take a floating point
argument with suffix in [kKMG], but this is not documented.

Update the documentation of the flux-ping(1) --pad option.

Author: grondo

doc/man1/flux-ping.rst

@@ -40,9 +40,11 @@ OPTIONS
    respectively. Default: send to “*any*”.
 
 **-p, --pad**\ *=N*
-   Include in the payload a string of length *N* bytes. The payload will be
-   echoed back in the response. This option can be used to explore the
-   effect of message size on latency. Default: no padding.
+   Include in the payload a string of length *N* bytes. *N* may be a
+   floating point number with optional multiplicative suffix k,K=1024,
+   M=1024\*1024, or G=1024\*1024\*1024. The payload will be echoed back in
+   the response. This option can be used to explore the effect of message
+   size on latency. Default: no padding.
 
 **-i, --interval**\ *=N*
    Specify the delay, in seconds, between successive requests.