enum_snmp: Cleanup and support non-Meterpreter sessions

[bcoles]

Resolves Rubocop violations.

Adds documentation.

Adds references.

Adds Notes module meta information.

Adds support for non-Meterpreter sessions. (Note: tested on shell sessions. Not tested with PowerShell, but the Post APIs used by this module should should work with PowerShell)

Improves error handling, the lack of which warrants the bug label. Previously, on modern Windows (after Windows 2000), for low-privileged sessions, the module would fail to retrieve the Community Strings and Permitted Managers, falsely claiming that they didn't exist. This was wrong. They exist, but require elevated privileges to view. This has been resolved by checking first whether the key exists with registry_key_exist?.