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##### SUMMARY
The current documentation has this explanation for the `firstmatch`
parameter of the `lineinfile` action:

```
If set, insertafter and insertbefore find a first line has regular expression matches.
```

I have no idea what `firstmatch` does, based on the above line.


##### ISSUE TYPE
- Documentation Report

##### COMPONENT NAME
lineinfile

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.4
  config file = None
  configured module search path = [u'/home/user/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /home/user/.virtualenvs/elasticluster/local/lib/python2.7/site-packages/ansible
  executable location = /home/user/.virtualenvs/elasticluster/bin/ansible
  python version = 2.7.14 (default, Sep 23 2017, 22:06:14) [GCC 7.2.0]
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below

```

module = AnsibleModule(
        argument_spec=dict(
            path=dict(type='path', required=True, aliases=['dest', 'destfile', 'name']),
            state=dict(type='str', default='present', choices=['absent', 'present']),
            regexp=dict(type='str', aliases=['regex']),
            line=dict(type='str', aliases=['value']),
            insertafter=dict(type='str'),
            insertbefore=dict(type='str'),
            backrefs=dict(type='bool', default=False),
            create=dict(type='bool', default=False),
            backup=dict(type='bool', default=False),
            firstmatch=dict(type='bool', default=False),
            validate=dict(type='str'),
        ),
        mutually_exclusive=[['insertbefore', 'insertafter']],
        add_file_common_args=True,
        supports_check_mode=True,
    )
    params = module.params
    create = params['create']
    backup = params['backup']
    backrefs = params['backrefs']
    path = params['path']