documentation enhancement for nxos syntax

[FliesLikeABrick]

Thank you for your assistance in #248. I am submitting this feature request regarding the nxos syntax mentioned in the resolution to that report.

Should the documentation at http://www.pennington.net/py/ciscoconfparse/api_CiscoConfParse.html be updated to mention the "nxos" syntax option? Ideally explain some ways in which this differs from "ios" in its parsing, so that other IOS/NX-OS-like syntaxes can be interpreted with the correct option.

It looks like perhaps other options may benefit from explanation of their differences/behaviors as well:
ALL_VALID_SYNTAX = (
'ios',
'nxos',
'asa',
'junos',
'terraform',
)

Thank you