Render custom tag briefs for enum entries and constructor

[strangesource]

Custom tags brief not rendered for enum entries and constructors

Contrary to what the documentation on the CustomTagContentProvider suggests, the custom tag brief is not rendered for constructors and enum entries. Reported in #4255.

Changes

• Render content for custom tag briefs for enum entries and constructors
• Minimal unit test

Issues encountered

While setting up the unit tests i struggled with the assertion DSL. It doesn't seem fight to assert the whole page tree until the node I actually want to ensure is present, maybe you have some suggestion on how this could be improved.

[whyoleg]

Sorry for the delay with reviewing :(

The changes look good, but we can simplify the test further by using Style, which will enable us to easily locate all comments with that style.
I would not say that it is fully idiomatic, but it should simplify the test logic.

[whyoleg]

Overall, the test is correct, but it could be done a bit more cleanly. Let's create an additional Style object, add it via CustomTagContentProvider:

// new style
object TestCustomStyle: Style

class TestCustomTagContentProvider : CustomTagContentProvider {
    override fun isApplicable(customTag: CustomTagWrapper): Boolean {
        return customTag.name == "customTag"
    }

    override fun PageContentBuilder.DocumentableContentBuilder.contentForBrief(
        sourceSet: DokkaConfiguration.DokkaSourceSet,
        customTag: CustomTagWrapper
    ) {
        // add style
        comment(customTag.children.single(), styles = mainStyles + setOf(TestCustomStyle))
    }
}

And in that case, it's possible to find it and do an assertion rather easily:

val customTagContents = page.content.withDescendants().filter {
    it.style.contains(TestCustomStyle)
}.toList()

assertEquals(1, customTagContents.size)
customTagContents.single().assertNode {
    group {
        +"custom tag for constructor"
    }
}

[whyoleg]

Would you also mind adding a test for class documentation and some function documentation with custom tags, so that we have all cases covered in one test?