[2.3][Form] renamed option "always_empty" to "reset_on_submit"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #6325
| License       | MIT
| Doc PR        | symfony/symfony-docs#3781

This PR renames the option "always_empty" to "reset_on_submit" for more clarity (the old option is deprecated and usable until 2.3).

The documentation PR is based on 2.6. It would be better to put it on 2.3.
shouldn't you retain the previous test case as well to prove that the BC support works?