Fix config diff logic if parent configuration is present more than once in the candidate config and update docs

[ganeshrn]

SUMMARY

If parent section present multiple times in the candidate configuration the diff is not generated correctly and
mainly applicable for <nos>_config modules

ISSUE TYPE

• Bug Report

COMPONENT NAME

ios_config
iosxr_config
eos_config
nxos_config
vyos_config

ANSIBLE VERSION

$ansible --version
ansible 2.9.16rc1

CONFIGURATION

OS / ENVIRONMENT

STEPS TO REPRODUCE

---
- hosts: ios
  gather_facts: false
  connection: ansible.netcommon.network_cli
  tasks:
    - cisco.ios.ios_config:
        src: config.cfg
      register: result

$cat  config.cfg
interface loopback0
 ip ospf 1 area 0.0.0.0
!
interface GigabitEthernet0/1
 ip ospf 1 area 0.0.0.0
interface GigabitEthernet0/2
 ip ospf 1 area 0.0.0.0
!
router isis fabric
 is-type level-2
 net 49.0000.0000.0003.00
!
interface loopback0
 ip router isis fabric
!
interface GigabitEthernet0/1
 ip router isis fabric
interface GigabitEthernet0/2
 ip router isis fabric

Running config on the device for relevant sections

interface Loopback0
 ip address 192.168.0.1 255.255.255.0
!         
interface GigabitEthernet0/0
 ip address 192.168.0.2 255.255.255.0
 duplex auto
 speed auto
 media-type rj45
!         
interface GigabitEthernet0/1
 ip address 192.168.0.3 255.255.255.0
 shutdown 
 duplex auto
 speed auto
 media-type rj45
!         
interface GigabitEthernet0/2
 ip address 192.168.0.4 255.255.255.0
 shutdown 
 duplex auto
 speed auto
 media-type rj45

EXPECTED RESULTS

Expected commands in the output

    "commands": [
        "interface loopback0",
        "ip ospf 1 area 0.0.0.0",
        "interface GigabitEthernet0/1",
        "ip ospf 1 area 0.0.0.0",
        "interface GigabitEthernet0/2",
        "ip ospf 1 area 0.0.0.0",
        "router isis fabric",
        "is-type level-2",
        "net 49.0000.0000.0003.00",
        "interface loopback0",
        "ip router isis fabric",
        "interface GigabitEthernet0/1",
        "ip router isis fabric",
        "interface GigabitEthernet0/2",
        "ip router isis fabric"
    ],

ACTUAL RESULTS

    "commands": [
        "interface loopback0",
        "ip ospf 1 area 0.0.0.0",
        "interface GigabitEthernet0/1",
        "ip ospf 1 area 0.0.0.0",
        "interface GigabitEthernet0/2",
        "ip ospf 1 area 0.0.0.0",
        "router isis fabric",
        "is-type level-2",
        "net 49.0000.0000.0003.00",
        "interface loopback0",
        "ip router isis fabric",
        "ip router isis fabric",
        "ip router isis fabric"
    ],