Confluence docs. Initial version

docs/confluence.jetbrains.com/Kotlin/40699414_Nested+classes.confluence

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+Classes can be nested in other classes:
+{jet}
+class Outer() {
+  private val bar : Int = 1
+  class Nested() {
+    fun foo() = 2
+  }
+}
+
+val demo = Outer.Inner().foo() // == 2
+{jet}
+
+h3. Inner classes
+
+A class may be marked as *inner* to be able to access members of outer class. Inner classes carry a reference to an object of an outer class:
+
+{jet}
+class Outer() {
+  private val bar : Int = 1
+  inner class Inner() {
+    fun foo() = bar
+  }
+}
+
+val demo = Outer().Inner().foo() // == 1
+{jet}
+
+See [Qualified *this* expressions|This expressions#Qualified] to learn about disambiguation of *this* in inner classes.
+
+{note:title=Inner classes are under development}See the corresponding [issue|http://youtrack.jetbrains.com/issue/KT-1174].{note}
+
+h3. What's next
+
+* [Object expressions and Declarations]