gcp_filestore_instance/networks/mode field not documented

##### SUMMARY

I'm trying to create a filestore instance in a already created private service connection.

The only option that shows up is MODE_IPV4
```
networks:
- network: default
modes:
- MODE_IPV4
```
I need to use the connect mode PRIVATE_SERVICE_ACCESS, but its not available or does not accept it.

##### ISSUE TYPE
- Documentation Report

##### COMPONENT NAME

##### ANSIBLE VERSION

```paste below
ha85006$ ansible --version
ansible 2.10.15
config file = /Users/#######/gitrepos/ansible-playbooks/ansible-role-gcp-nfs/tests/ansible.cfg
configured module search path = ['/Users/#####/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
ansible python module location = /Users/####/Library/Python/3.9/lib/python/site-packages/ansible
executable location = /Users/######/Library/Python/3.9/bin/ansible
python version = 3.9.4 (default, Apr 5 2021, 01:49:30) [Clang 12.0.0 (clang-1200.0.32.29)]
```

Start with the gcp_filestore_instance module documentation and the networks/mode field described in the issue. Verify whether PRIVATE_SERVICE_ACCESS is supported alongside MODE_IPV4, then update the documentation so the available value and its usage are clear; done when the documented options match the module's behavior.

Written by the indexing model from the issue text.