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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

```

'''
EXAMPLES = r'''
# NOTE: Before 2.3, option 'dest', 'destfile' or 'name' was used instead of 'path'
- name: Ensure SELinux is set to enforcing mode
  lineinfile:
    path: /etc/selinux/config
    regexp: '^SELINUX='
    line: SELINUX=enforcing
- name: Make sure group wheel is not in the sudoers configuration
  lineinfile:
    path: /etc/sudoers
    state: absent
    regexp: '^%wheel'
- name: Replace a localhost entry with our own
  lineinfile:
    path: /etc/hosts
    regexp: '^127\.0\.0\.1'
    line: 127.0.0.1 localhost
    owner: root
    group: root
    mode: '0644'
- name: Ensure the default Apache port is 8080
  lineinfile:
    path: /etc/httpd/conf/httpd.conf
    regexp: '^Listen '
    insertafter: '^#Listen '
    line: Listen 8080
- name: Ensure we have our own comment added to /etc/services
  lineinfile:
    path: /etc/services