docs: Document newly introduced net_dhcp command

docs: Document newly introduced net_dhcp command

Commit 5bc41db756c5 ("net/dhcp: Add explicit net_dhcp command")
introduced the new command "net_dhcp", which (for now) is an alias for
the existing "net_bootp". Unfortunately the TEXI documentation was not
adjusted accordingly.

Rename the existing paragraph about net_bootp to read net_dhcp instead,
and make the net_bootp stanza point to this new command.

On the way add the newly parsed TFTP_SERVER_NAME and BOOTFILE_NAME
packets to the list of supported DHCP options.

Fixes bug: https://savannah.gnu.org/bugs/?56725

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>