[IR] Add kdoc for IrExternalPackageFragment

compiler/ir/ir.tree/gen/org/jetbrains/kotlin/ir/declarations/IrExternalPackageFragment.kt

@@ -15,7 +15,21 @@ import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
 import org.jetbrains.kotlin.serialization.deserialization.descriptors.DeserializedContainerSource
 
 /**
- * A leaf IR tree element.
+ * This is a root parent element for external declarations (meaning those that come from
+ * another compilation unit/module, not to be confused with [IrPossiblyExternalDeclaration.isExternal]). 
+ *
+ * Each declaration is contained either in some [IrFile], or in some [IrExternalPackageFragment].
+ * Declarations coming from dependencies are located in [IrExternalPackageFragment].
+ *
+ * It can be used for obtaining a module descriptor, which contains the information about
+ * the module from which the declaration came. It would be more correct to have a link to some
+ * [IrModuleFragment] instead, which would make [IrModuleFragment] the only source of truth about modules,
+ * but this is how things are now.
+ *
+ * Also, it can be used for checking whether some declaration is external (by checking whether its top
+ * level parent is an [IrExternalPackageFragment]). But it is not possible
+ * to get all declarations from a fragment. Also, being in the same or different
+ * fragment doesn't mean anything. There can be more than one fragment for the same dependency.
  *
  * Generated from: [org.jetbrains.kotlin.ir.generator.IrTree.externalPackageFragment]
  */

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/IrTree.kt

@@ -625,6 +625,24 @@ object IrTree : AbstractTreeBuilder() {
         transformByChildren = true
         generateIrFactoryMethod = false
 
+        kDoc = """
+            This is a root parent element for external declarations (meaning those that come from
+            another compilation unit/module, not to be confused with [IrPossiblyExternalDeclaration.isExternal]). 
+            
+            Each declaration is contained either in some [IrFile], or in some [IrExternalPackageFragment].
+            Declarations coming from dependencies are located in [IrExternalPackageFragment].
+            
+            It can be used for obtaining a module descriptor, which contains the information about
+            the module from which the declaration came. It would be more correct to have a link to some
+            [IrModuleFragment] instead, which would make [IrModuleFragment] the only source of truth about modules,
+            but this is how things are now.
+            
+            Also, it can be used for checking whether some declaration is external (by checking whether its top
+            level parent is an [IrExternalPackageFragment]). But it is not possible
+            to get all declarations from a fragment. Also, being in the same or different
+            fragment doesn't mean anything. There can be more than one fragment for the same dependency.
+        """.trimIndent()
+
         parent(packageFragment)
 
         +symbol(externalPackageFragmentSymbolType)