docs: fix outdated statement that Docker major updates are disabled by default

[aryan-barsaiya]

Changes

docs/usage/docker.md still says that Renovate will not upgrade major versions of Docker images by default, and recommends adding docker:enableMajor to enable them.

This has been outdated since #9470 (April 2021, Renovate v25), which removed major: { enabled: false } from the docker datasource's defaultConfig. Since then, major updates for the docker datasource are enabled by default, and neither config:recommended nor any preset it extends disables them (docker:disableMajor / docker:enableMajor are both opt-in).

This PR updates the two affected spots:

• The "Version Upgrading" paragraph now states that major, minor and patch are upgraded by default, and points to docker:disableMajor for opting out.
• The "Enable Docker major updates" config section is replaced with "Disable Docker major updates" (docker:disableMajor), since enabling is already the default.

The stale wording actively misleads users: we saw an automated code reviewer cite this page to (incorrectly) flag a packageRules entry matching matchUpdateTypes: ["major"] on a docker datasource as dead code.

Context

Please select one of the following:

• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

• No — I did not use AI for this contribution.
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• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

This PR (research, wording, and commit) was prepared with Claude Code (model: Claude Fable 5), operated and reviewed by me. The root cause was verified against the source: the docker datasource defaultConfig on main and v43, the config:recommended preset chain, and the #9470 diff.

Documentation (please check one with an [x])

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• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository

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