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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 regexp is not None:
            match_found = bre_c.search(b_cur_line)
        else:
            match_found = b_line == b_cur_line.rstrip(b'\r\n')
        if match_found:
            found.append(b_cur_line)
        return not match_found
    b_lines = [l for l in b_lines if matcher(l)]
    changed = len(found) > 0
    if module._diff:
        diff['after'] = to_native(b''.join(b_lines))
    backupdest = ""
    if changed and not module.check_mode:
        if backup:
            backupdest = module.backup_local(dest)
        write_changes(module, b_lines, dest)
    if changed:
        msg = "%s line(s) removed" % len(found)
    attr_diff = {}
    msg, changed = check_file_attrs(module, changed, msg, attr_diff)
    attr_diff['before_header'] = '%s (file attributes)' % dest
    attr_diff['after_header'] = '%s (file attributes)' % dest
    difflist = [diff, attr_diff]
    module.exit_json(changed=changed, found=len(found), msg=msg, backup=backupdest, diff=difflist)
def main():