Pull Interfaces and Abstract contracts below a common docs section

docs/contracts.rst

@@ -1048,13 +1048,22 @@ When the inheritance results in a contract with a function and a modifier of the
 This error is produced also by an event and a modifier of the same name, and a function and an event of the same name.
 As an exception, a state variable getter can override a public function.
 
+.. index:: ! abstractions
+
+.. _abstractions:
+
+************
+Abstractions
+************
+
+An abstraction type describes a contract but does not provide a full implementation of the contract. An abstraction is implemented as either an abstract contract or interface.
+
 .. index:: ! contract;abstract, ! abstract contract
 
 .. _abstract-contract:
 
-******************
 Abstract Contracts
-******************
+==================
 
 Contracts are marked as abstract when at least one of their functions lacks an implementation as in the following example (note that the function declaration header is terminated by ``;``)::
 
@@ -1080,9 +1089,8 @@ If a contract inherits from an abstract contract and does not implement all non-
 
 .. index:: ! contract;interface, ! interface contract
 
-**********
 Interfaces
-**********
+==========
 
 Interfaces are similar to abstract contracts, but they cannot have any functions implemented. There are further restrictions: