docs: Clearer wording in SPA notice for HTML forms

[louis-lau]

Related issue

https://ory-community.slack.com/archives/C012RBW0F18/p1621977892051700

Proposed changes

As discussed in slack @vinckr, a little clearer wording for the notice about HTML forms in the hydra docs.

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

I edited the notice in all doc versions, let me know if you want it done another way :)

[CLAassistant]

All committers have signed the CLA.

[vinckr]

Thank you for the contribution 🎉 !

To make the format check pass, could you please run:
cd docs; npm i; npm run format
from the hydra main dir?

In the future we could consider using something like the APIWarning for Kratos here as well, it should make editing this easier, but I don't think it is necessary for this PR.

[louis-lau]

Good one, I've amended my commit, it's formatted with prettier now :)

[aeneasr]

Thank you! :)