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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

```

description:
      - Whether the line should be there or not.
    type: str
    choices: [ absent, present ]
    default: present
  line:
    description:
      - The line to insert/replace into the file.
      - Required for C(state=present).
      - If C(backrefs) is set, may contain backreferences that will get
        expanded with the C(regexp) capture groups if the regexp matches.
    type: str
    aliases: [ value ]
  backrefs:
    description:
      - Used with C(state=present).
      - If set, C(line) can contain backreferences (both positional and named)
        that will get populated if the C(regexp) matches.
      - This parameter changes the operation of the module slightly;
        C(insertbefore) and C(insertafter) will be ignored, and if the C(regexp)
        does not match anywhere in the file, the file will be left unchanged.
      - If the C(regexp) does match, the last matching line will be replaced by
        the expanded line parameter.
    type: bool
    default: no
    version_added: "1.1"
  insertafter:
    description:
      - Used with C(state=present).
      - If specified, the line will be inserted after the last match of specified regular expression.