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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

```

pass
    
    elif insertbefore == 'BOF' or insertafter == 'BOF':
        b_lines.insert(0, b_line + b_linesep)
        msg = 'line added'
        changed = True
    
    
    
    elif insertafter == 'EOF' or index[1] == -1:
        
        if b_lines and not b_lines[-1][-1:] in (b'\n', b'\r'):
            b_lines.append(b_linesep)
        b_lines.append(b_line + b_linesep)
        msg = 'line added'
        changed = True
    elif insertafter and index[1] != -1:
        
        if b_line != b_lines[index[1]].rstrip(b'\n\r'):
            b_lines.insert(index[1], b_line + b_linesep)
            msg = 'line added'
            changed = True
    
    else:
        b_lines.insert(index[1], b_line + b_linesep)
        msg = 'line added'
        changed = True
    if module._diff:
        diff['after'] = to_native(b''.join(b_lines))
    backupdest = ""