⚠️ Strip comments from CRD descriptions

[tsaarni]

This PR strips comments from the generated CRDs for clarity of the public API documentation.

The change aligns controller-tools with Kubernetes conventions for type descriptions:

• Line that only contains --- separates the API description from comments. The leading lines are part of public documentation, the trailing lines are internal instructions such as code examples etc. As an example, see widely used type Condition that includes some example code that should not be considered as part of API documentation.
• Line that begins with TODO is internal note for implementer, and not part of public documentation of the type. See LocalObjectReference for example.

Background information

When Kubernetes itself generates OpenAPI docs for its internal resources, it will strip the implementation related comments from the go docs. I believe it is done by this code.

However with controller-tools the implementation comments "leak" to public documentation. This causes confusion and usability issues since the users have no context to understand the implementation comments.

Google search for "Represents the observations of a foo's current state" shows that the problem is wide spread.

Limited compatibility with markdown formatting

I've marked the PR as ⚠️ breaking with following reasoning:

While I first thought it is very unlikely to bump into --- in type's documentation, I'm proven wrong by #870 which shows that sometimes markdown formatting is used inside go docs. kubectl explain will not understand how to format markdown, but the reasoning for markdown inside go docs might come from OpenAPI spec which supports "rich text formatting" with CommonMark 0.27

There is a limitation: There is no reliable way to know if go doc is in plain text or markdown, therefore --- could mean comment separator (according to Kubernetes convention) or alternatively thematic break or heading (according to markdown). The implementation takes an approach where --- inside markdown fenced code blocks is kept, but when seen in plain, it is considered as comment separator and rest of the doc is stripped. This might be wrong assumption in some corner cases. As result the description in the generated CRD will be cut short.

Fixes #649, #728, #875

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[tsaarni]

I'm cc'ing @qinqon, @cbandy, @kaovilai in case you are interested to comment this PR as well.

[kaovilai]

For my use this is lgtm

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: kaovilai, tsaarni
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[qinqon]

Since it discriminates between code blocks and "normal" --- is fine by me
/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 037e0f10c0dbb0835758444fadb42fe99470ea1c

[tsaarni]

/assign @vincepri

[tsaarni]

Hi @vincepri, If you have a moment, could you please take a look? Your feedback would be greatly appreciated 🙏