Add distinction for platform-specific types

proposals/p2015.md

@@ -144,6 +144,9 @@ Which expands to:
     `Carbon.UInt(N={8,16,32,64,128})`
 -   `f16`, `f32`, and `f64` keyword aliases for `Carbon.Float(N={16,32,64})`
 
+Additionally, other type sizes may be available, depending on the platform (for
+example, `x85`), such as `f80`, `f128`, or `f256`.
+
 #### Syntax example use in arbitrary fixed-size numeric types
 
 In the more general case, this syntax can express N-width types.
@@ -169,6 +172,15 @@ Highlighting relevant aspects of this from the project goals:
     ceremony add cognitive overhead for the reader, while explicitness reduces
     the amount of outside context the reader must have or assume._
 
+The type system syntax must also complement Carbon's target for
+["Performance-critical software"](https://github.com/carbon-language/carbon-lang/blob/trunk/docs/project/goals.md#performance-critical-software)
+
+Specifically, there should be "No need for a lower level language."
+
+-   _Developers should not need to leave the rules and structure of Carbon,
+    whether to gain control over performance problems or to gain access to
+    hardware facilities._
+
 ## Alternatives considered
 
 As discussed in