[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.
Thanks, I rebased to readd the original test.