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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

```

regexp: '^# port for http'
    insertbefore: '^www.*80/tcp'
    line: '# port for http by default'
- name: Add a line to a file if the file does not exist, without passing regexp
  lineinfile:
    path: /tmp/testfile
    line: 192.168.1.99 foo.lab.net foo
    create: yes
# NOTE: Yaml requires escaping backslashes in double quotes but not in single quotes
- name: Ensure the JBoss memory settings are exactly as needed
  lineinfile:
    path: /opt/jboss-as/bin/standalone.conf
    regexp: '^(.*)Xms(\\d+)m(.*)$'
    line: '\1Xms${xms}m\3'
    backrefs: yes
# NOTE: Fully quoted because of the ': ' on the line. See the Gotchas in the YAML docs.
- name: Validate the sudoers file before saving
  lineinfile:
    path: /etc/sudoers
    state: present
    regexp: '^%ADMIN ALL='
    line: '%ADMIN ALL=(ALL) NOPASSWD: ALL'
    validate: /usr/sbin/visudo -cf %s
'''
import os
import re
import tempfile
from ansible.module_utils.basic import AnsibleModule
from ansible.module_utils._text import to_bytes, to_native
def write_changes(module, b_lines, dest):