Document network-boot-arguments for sparc64 diskless(8)

Document network-boot-arguments for sparc64 diskless(8)

The Oracle OpenBoot 4.x Administration Guide[0] documents a few useful
options for network boot.

Basically, either
	{ok} setenv network-boot-arguments tftp-retries=0
	{ok} boot net ...
or
	{ok} boot net:tftp-retries=0 ...

Newer machines like the T4-2 using OpenBoot 4.38.16 also support BOOTP/DHCP
besides RARP:  "dhcp,hostname=puffy" works as expected on my
T4-2, but "boot-filename" and "boot-retries" remain unsupported on this
particular machine despite being documented in the guide.

0: https://docs.oracle.com/cd/E63648_01/html/E63649/gpvll.html#scrolltoc

OK miod