Add docs

Author: odersky

docs/docs/reference/metaprogramming/inline.md

@@ -142,6 +142,32 @@ funkyAssertEquals(computeActual(), computeExpected(), computeDelta())
 //    if (actual - expected).abs > computeDelta() then
 //      throw new AssertionError(s"difference between ${expected} and ${actual} was larger than ${computeDelta()}")
 ```
+### Rules for Overriding
+
+Inline methods can override other methods and can themselves be overridden by other inline methods. The rules are as follows:
+
+1. If an inline method `f` implements or overrides another, non-inline method, the inline method can also be invoked at runtime. For instance, consider the scenario:
+```scala
+abstract class A {
+  def f(): Int
+  def g(): Int = f()
+}
+class B extends A {
+  inline def f() = 22
+  override inline def g() = f() + 11
+}
+val b = B()
+val a: A = b
+// inlined invocatons
+assert(b.f() == 22)
+assert(b.g() == 33)
+// dynamic invocations
+assert(a.f() == 22)
+assert(a.g() == 33)
+```
+The inlined invocations and the dynamically dispatched invocations give the same results.
+
+2. Inline methods can override or implement normal methods, as the previous example shows. Inline methods can also be abstract. Inline methods can be overridden or implemented only by other inline methods.
 
 ### Relationship to @inline
 
@@ -191,7 +217,7 @@ numeric computations.
 
 ## Specializing Inline (Whitebox)
 
-Inline methods support the ` <: T` return type syntax. This means that the return type
+Inline methods support the `<: T` return type syntax. This means that the return type
 of the inline method is going to be specialized to a more precise type upon
 expansion. Example: