[docs] Add "currency type" to the lexicon (swiftlang#40874)

* Unclear word

* Update Lexicon.md

* Update README.md

* Update docs/Lexicon.md

Co-authored-by: Robert Widmann <devteam.codafi@gmail.com>

* Update Lexicon.md

* Update docs/README.md

Co-authored-by: Robert Widmann <devteam.codafi@gmail.com>

* Update docs/README.md

Co-authored-by: Xiaodi Wu <13952+xwu@users.noreply.github.com>

Co-authored-by: Robert Widmann <devteam.codafi@gmail.com>
Co-authored-by: Xiaodi Wu <13952+xwu@users.noreply.github.com>

Author: 3 people

docs/Lexicon.md

@@ -137,6 +137,19 @@ the AST level. See also [witness table](#witness-table).
 An edge in a control flow graph where the destination has multiple predecessors
 and the source has multiple successors.
 
+## currency type
+
+A type that's meant to be commonly passed around and stored, like `Array`, as
+opposed to a type that's useful for temporary/internal purposes but which you
+wouldn't normally use in an external interface, like `ArraySlice`. Having broad
+agreement about the currency type you use for a particular kind of data (e.g.
+using `Array` to pass around sequential collections) generally makes the whole
+ecosystem better by reducing artificial barriers to passing data from one system 
+to another, and it gives algorithm writers an obvious target to ensure they
+optimize for. That's where the analogy to currency comes from: agreeing on a
+currency type improves the flow of information in a program in some of the same
+ways that agreeing on a currency improves the flow of trade in an economy.
+
 ## customization point
 
 Informal term for a protocol requirement that has a default implementation,

docs/README.md

@@ -131,7 +131,8 @@ documentation, please create a thread on the Swift forums under the
     Describes the design of the optimizer pipeline.
   - [HighLevelSILOptimizations.rst](/docs/HighLevelSILOptimizations.rst):
     Describes how the optimizer understands the semantics of high-level
-    operations on currency data types and optimizes accordingly.
+    operations on [currency](/docs/Lexicon.md#currency-type) data types and 
+    optimizes accordingly.
     Includes a thorough discussion of the `@_semantics` attribute.
 
 ### SourceKit subsystems