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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

```

index[1] = lineno + 1
                    if firstmatch:
                        break
                if insertbefore:
                    
                    index[1] = lineno
                    if firstmatch:
                        break
    msg = ''
    changed = False
    b_linesep = to_bytes(os.linesep, errors='surrogate_or_strict')
    
    if index[0] != -1:
        if backrefs:
            b_new_line = m.expand(b_line)
        else:
            
            b_new_line = b_line
        if not b_new_line.endswith(b_linesep):
            b_new_line += b_linesep
        
        
        if regexp is None and m is None:
            
            if insertafter and insertafter != 'EOF':
                
                
                if b_lines and not b_lines[-1][-1:] in (b'\n', b'\r'):
                    b_lines[-1] = b_lines[-1] + b_linesep