Remove misleading statement passwords must be same (#49798)

* Remove misleading statement passwords must be same

Since 2.4 Ansible has supported multiple vault passwords:
<https://docs.ansible.com/ansible/latest/user_guide/vault.html#multiple-vault-passwords>

Meaning lines like the following are misleading:

> The password used with vault currently must be the same for all files you wish
> to use together at the same time.

-- `docs/docsite/rst/user_guide/vault.rst`

To demonstrate this with Ansible 2.7, save the following as `example.yaml`:

```
- name: Display output from two vaults with different passwords
  hosts: localhost
  connection: local
  vars_files: [one.yaml, two.yaml]
  tasks:
    - name: View secret from one.yaml vault
      debug: { var: one }
    - name: View secret from two.yaml vault
      debug: { var: two }
```

Then run the three following commands choosing two different passwords:

```
$ echo 'one: 1' | ansible-vault encrypt --vault-id id1@prompt --output=one.yaml
$ echo 'two: 2' | ansible-vault encrypt --vault-id id2@prompt --output=two.yaml
$ ansible-playbook --vault-id id1@prompt --vault-id id2@prompt example.yaml
```

`ansible-vault` stores an ID in plain text in the vault file.

* Remove note about default in Ansible 2.1

As requested by gundalow in https://github.com/ansible/ansible/pull/49798

lib/ansible/cli/vault.py

@@ -30,8 +30,6 @@ class VaultCLI(CLI):
 
     Because Ansible tasks, handlers, and other objects are data, these can also be encrypted with vault.
     If you'd like to not expose what variables you are using, you can keep an individual task file entirely encrypted.
-
-    The password used with vault currently must be the same for all files you wish to use together at the same time.
     '''
 
     VALID_ACTIONS = frozenset(("create", "decrypt", "edit", "encrypt", "encrypt_string", "rekey", "view"))