Add external Etcd options to docs

[jnummelin]

Description

The implementation has been done already a while ago in #1165 but the docs never got updated to reflect that 😢

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[jnummelin]

@twz123 fixed the suggestions, PTAL

[k0s-bot]

Successfully created backport PR for release-1.26:

• [Backport release-1.26] Add external Etcd options to docs #3944

[k0s-bot]

Successfully created backport PR for release-1.27:

• [Backport release-1.27] Add external Etcd options to docs #3945

[k0s-bot]

Successfully created backport PR for release-1.28:

• [Backport release-1.28] Add external Etcd options to docs #3946