Added documentation for System.Net.Sockets APIs targeted for 3.0 (#2767)

* Added documentation for System.Net.Sockets APIs targeted for 3.0

* Addressed comments

Removed "%2A" from properties.

On ReadAsync(System.Memory{System.Byte},System.Threading.CancellationToken), removed unnecessary exceptions.

* Addressing more commnets:
* standarizing summaries of SendPacketElements.ctors

* Adding note to SafeSocketHandle, related to dotnet/corefx#39677

Author: jozkee

xml/System.Net.Sockets/AddressFamily.xml

@@ -300,7 +300,7 @@
       </ReturnValue>
       <MemberValue>32</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Controller Area Network address.</summary>
       </Docs>
     </Member>
     <Member MemberName="DataKit">
@@ -920,7 +920,7 @@
       </ReturnValue>
       <MemberValue>30</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Netlink address.</summary>
       </Docs>
     </Member>
     <Member MemberName="NetworkDesigners">
@@ -1060,7 +1060,7 @@
       </ReturnValue>
       <MemberValue>31</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Low-level Packet address.</summary>
       </Docs>
     </Member>
     <Member MemberName="Pup">

xml/System.Net.Sockets/NetworkStream.xml

@@ -261,7 +261,7 @@
       </ReturnValue>
       <MemberValue>32</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Controller Area Network protocol.</summary>
       </Docs>
     </Member>
     <Member MemberName="DataKit">
@@ -815,7 +815,7 @@
       </ReturnValue>
       <MemberValue>30</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Netlink protocol.</summary>
       </Docs>
     </Member>
     <Member MemberName="NetworkDesigners">
@@ -940,7 +940,7 @@
       </ReturnValue>
       <MemberValue>31</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Low-level Packet protocol.</summary>
       </Docs>
     </Member>
     <Member MemberName="Pup">

xml/System.Net.Sockets/ProtocolFamily.xml

@@ -14,8 +14,16 @@
   </Base>
   <Interfaces />
   <Docs>
-    <summary>To be added.</summary>
-    <remarks>To be added.</remarks>
+    <summary>Represents a wrapper class for a socket handle.</summary>
+    <remarks>
+      <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+
+ This class is derived from <xref:Microsoft.Win32.SafeHandles.SafeHandleZeroOrMinusOneIsInvalid>. A value of 0 or -1 is an invalid socket handle.  
+  
+ ]]></format>
+    </remarks>
   </Docs>
   <Members>
     <Member MemberName=".ctor">
@@ -35,9 +43,9 @@
         <Parameter Name="ownsHandle" Type="System.Boolean" Index="1" FrameworkAlternate="netcore-3.0" />
       </Parameters>
       <Docs>
-        <param name="preexistingHandle">To be added.</param>
-        <param name="ownsHandle">To be added.</param>
-        <summary>To be added.</summary>
+        <param name="preexistingHandle">The pre-existing handle to use.</param>
+        <param name="ownsHandle"><see langword="true" /> to reliably release the handle during the finalization phase; <see langword="false" /> to prevent reliable release (not recommended).</param>
+        <summary>Initializes a new instance of the <see cref="T:System.Net.Sockets.SafeSocketHandle" /> class.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.Net.Sockets/SafeSocketHandle.xml

@@ -8341,9 +8341,16 @@ The GetHashCode method returns a hash code of this instance. This value can be u
         <ReturnType>System.Net.Sockets.SafeSocketHandle</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>To be added.</summary>
-        <value>To be added.</value>
-        <remarks>To be added.</remarks>
+        <summary>Gets a <see cref="T:System.Net.Sockets.SafeSocketHandle" /> that represents the socket handle that the current <see cref="T:System.Net.Sockets.Socket" /> object encapsulates.</summary>
+        <value>A socket handle exposed in a safe manner for the socket that the current <see cref="T:System.Net.Sockets.Socket" /> object encapsulates.</value>
+        <remarks>  
+          <format type="text/markdown"><![CDATA[  
+
+> [!IMPORTANT]
+> Using the <xref:System.Net.Sockets.SafeSocketHandle> to operate on the socket directly may result in an undocumented state. 
+
+         ]]></format>
+         </remarks>
       </Docs>
     </Member>
     <Member MemberName="Select">

xml/System.Net.Sockets/SendPacketsElement.xml

@@ -1425,7 +1425,7 @@
       </ReturnValue>
       <MemberValue>17</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The number of seconds a TCP connection will wait for a keepalive response before sending another keepalive probe.</summary>
       </Docs>
     </Member>
     <Member MemberName="TcpKeepAliveRetryCount">
@@ -1451,7 +1451,7 @@
       </ReturnValue>
       <MemberValue>16</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The number of TCP keep alive probes that will be sent before the connection is terminated.</summary>
       </Docs>
     </Member>
     <Member MemberName="TcpKeepAliveTime">
@@ -1477,7 +1477,7 @@
       </ReturnValue>
       <MemberValue>3</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The number of seconds a TCP connection will remain alive/idle before keepalive probes are sent to the remote.</summary>
       </Docs>
     </Member>
     <Member MemberName="Type">