refactor: add on-chain program (solana-foundation#36)

* refactor: add on-chain program

Per other Foundation / Labs communication.

* fix bad link text in terminology, onchain is apparently one word

docs/terminology.md

@@ -280,6 +280,10 @@ A computer participating in a [cluster](#cluster).
 
 The number of [validators](#validator) participating in a [cluster](#cluster).
 
+## onchain program
+
+A program on a blockchain that can read and modify accounts over which it has control.
+
 ## PoH
 
 See [Proof of History](#proof-of-history-poh).
@@ -411,8 +415,7 @@ non-overlapping, comprising roughly equal real-world time as per
 
 ## smart contract
 
-A program on a blockchain that can read and modify accounts over which it has
-control.
+See [onchain program](#onchain-program).
 
 ## sol