Add per-architecture availability note.

The documentation publication pipeline doesn't have a way to expose
per-architecture availability, like how it it shows per-platform
availability.  So add this in prose instead.

Fixes <rdar://77649895>

Author: amartini51

stdlib/public/core/FloatingPointTypes.swift.gyb

@@ -41,8 +41,9 @@ def Availability(bits):
 
 if Self == 'Float16':
     SelfDocComment = '''\
-/// A half-precision (16b), floating-point value type.'''
-
+/// A half-precision (16b), floating-point value type.
+///
+/// `Float16` is available on Apple silicon, and unavailable on Intel.'''
 elif Self == 'Float':
     SelfDocComment = '''\
 /// A single-precision, floating-point value type.'''
@@ -53,7 +54,9 @@ elif Self == 'Double':
 
 elif Self == 'Float80':
     SelfDocComment = '''\
-/// An extended-precision, floating-point value type.'''
+/// An extended-precision, floating-point value type.
+///
+/// `Float80` is available on Intel, and unavailable on Apple silicon.'''
 
 else:
     raise ValueError('Unhandled float type.')