networking: Documentation: fix snmp_counters.rst Sphinx warnings

Fix over 100 documentation warnings in snmp_counter.rst by
extending the underline string lengths and inserting a blank line
after bullet items.

Examples:

Documentation/networking/snmp_counter.rst:1: WARNING: Title overline too short.
Documentation/networking/snmp_counter.rst:14: WARNING: Bullet list ends without a blank line; unexpected unindent.

Fixes: 2b965472 ("add document for TCP OFO, PAWS and skip ACK counters")
Fixes: 8e2ea53a ("add snmp counters document")
Fixes: 712ee16c ("add documents for snmp counters")
Fixes: 80cc4950 ("net: Add part of TCP counts explanations in snmp_counters.rst")
Fixes: b08794a9 ("documentation of some IP/ICMP snmp counters")

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: yupeng <yupeng0921@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>