Updates to empty namespace docs (#5055)

Author: stefanrbk

xml/ns-System.Runtime.Intrinsics.Arm.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.Runtime.Intrinsics.Arm">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Exposes select instruction-set extensions for ARM systems. These instruction sets are expressed as separate classes for each extension. Support for any extension within the current environment can be determined by querying the <see langword="IsSupported" /> property on the respective type.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>

xml/ns-System.Runtime.Intrinsics.X86.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.Runtime.Intrinsics.X86">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Exposes select instruction-set extensions for x86 and x64 systems. These instruction sets are expressed as separate classes for each extension. Support for any extension within the current environment can be determined by querying the <see langword="IsSupported" /> property on the respective type.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>

xml/ns-System.Runtime.Intrinsics.xml

@@ -1,6 +1,11 @@
 <Namespace Name="System.Runtime.Intrinsics">
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Contains types used to create and convey register states in various sizes and formats for use with instruction-set extensions.
+    
+For the instructions to manipulate these registers, see <see cref="N:System.Runtime.Intrinsics.X86" /> and <see cref="N:System.Runtime.Intrinsics.Arm" />.
+    </summary>
     <remarks>To be added.</remarks>
+    <altmember cref="N:System.Runtime.Intrinsics.X86" />
+    <altmember cref="N:System.Runtime.Intrinsics.Arm" />
   </Docs>
 </Namespace>