Symlink to relative paths in target directory (documentaion update); fix of incorrect usage of the force parameter in case of hard links

Dear Ansible Dev Team,

Here is the brief description of my small work:

- Update of the description of the 'force' parameter (file module now allows to create symlinks to non-existent desctinations)
- The addition of one example of creation the symlink pointing to non-absolute paths of type ```x -> y``` in target directory
- fixed incorrect usage of force parameter in case of hard links: src must always exists, otherwise non-correct error message is produced. For example, non-correct message (/tmp/yyy does not exist):
```
[root] # ansible 127.0.0.1 --connection=local -m file -a "path=/tmp/xxx src=/tmp/yyy state=hard force=yes"
127.0.0.1 | FAILED >> {
    "failed": true, 
    "msg": "Error while linking: [Errno 2] No such file or directory", 
    "path": "/tmp/xxx", 
    "state": "absent"
}
```
Patched version (/tmp/yyy does not exist):
```
[root@abpc11619] /var/lib/ansible # ansible 127.0.0.1 --connection=local -m file -a "path=/tmp/xxx src=/tmp/yyy state=hard force=yes"
127.0.0.1 | FAILED >> {
    "failed": true, 
    "msg": "src file does not exist", 
    "path": "/tmp/xxx", 
    "src": "/tmp/yyy", 
    "state": "absent"
}
```