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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

```

- This module ensures a particular line is in a file, or replace an
    existing line using a back-referenced regular expression.
  - This is primarily useful when you want to change a single line in a file only.
  - See the M(replace) module if you want to change multiple, similar lines
    or check M(blockinfile) if you want to insert/update/remove a block of lines in a file.
    For other cases, see the M(copy) or M(template) modules.
version_added: "0.7"
options:
  path:
    description:
      - The file to modify.
      - Before Ansible 2.3 this option was only usable as I(dest), I(destfile) and I(name).
    type: path
    required: true
    aliases: [ dest, destfile, name ]
  regexp:
    description:
      - The regular expression to look for in every line of the file.
      - For C(state=present), the pattern to replace if found. Only the last line found will be replaced.
      - For C(state=absent), the pattern of the line(s) to remove.
      - If the regular expression is not matched, the line will be
        added to the file in keeping with C(insertbefore) or C(insertafter)
        settings.
      - When modifying a line the regexp should typically match both the initial state of
        the line as well as its state after replacement by C(line) to ensure idempotence.
      - Uses Python regular expressions. See U(http://docs.python.org/2/library/re.html).
    type: str
    aliases: [ regex ]
    version_added: '1.7'
  state: