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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

```

- If the first match is required, use(firstmatch=yes).
      - A special value is available; C(EOF) for inserting the line at the end of the file.
      - If specified regular expression has no matches, EOF will be used instead.
      - If C(insertbefore) is set, default value C(EOF) will be ignored.
      - If regular expressions are passed to both C(regexp) and C(insertafter), C(insertafter) is only honored if no match for C(regexp) is found.
      - May not be used with C(backrefs) or C(insertbefore).
    type: str
    choices: [ EOF, '*regex*' ]
    default: EOF
  insertbefore:
    description:
      - Used with C(state=present).
      - If specified, the line will be inserted before the last match of specified regular expression.
      - If the first match is required, use C(firstmatch=yes).
      - A value is available; C(BOF) for inserting the line at the beginning of the file.
      - If specified regular expression has no matches, the line will be inserted at the end of the file.
      - If regular expressions are passed to both C(regexp) and C(insertbefore), C(insertbefore) is only honored if no match for C(regexp) is found.
      - May not be used with C(backrefs) or C(insertafter).
    type: str
    choices: [ BOF, '*regex*' ]
    version_added: "1.1"
  create:
    description:
      - Used with C(state=present).
      - If specified, the file will be created if it does not already exist.
      - By default it will fail if the file is missing.
    type: bool
    default: no
  backup:
    description: