Update ProductInfoHeaderValue.xml

Fixes dotnet#3274

Author: wenz

xml/System.Net.Http.Headers/ProductInfoHeaderValue.xml

@@ -44,7 +44,7 @@
       <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The <xref:System.Net.Http.Headers.ProductInfoHeaderValue> class provides support for the product and comment tokens values used in  a User-Agent header as defined in [RFC 2616](https://go.microsoft.com/fwlink/?LinkID=241642) by the IETF.  
+ The <xref:System.Net.Http.Headers.ProductInfoHeaderValue> class provides support for the product and comment tokens values used in a User-Agent header as defined in [RFC 2616](https://go.microsoft.com/fwlink/?LinkID=241642) by the IETF.  
   
  The User-Agent field contains information about the user agent originating the request. The field can contain multiple product tokens and comments identifying the user agent and any sub-products.  
   
@@ -93,7 +93,7 @@
       </Parameters>
       <Docs>
         <param name="product">A <see cref="T:System.Net.Http.Headers.ProductHeaderValue" /> object used to initialize the new instance.</param>
-        <summary>Initializes a new instance of the <see cref="T:System.Net.Http.Headers.ProductInfoHeaderValue" /> class.</summary>
+        <summary>Initializes a new instance of the <see cref="T:System.Net.Http.Headers.ProductInfoHeaderValue" /> class, using a product token from a <see cref="T:System.Net.Http.Headers.ProductHeaderValue" /> object.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -126,8 +126,15 @@
       </Parameters>
       <Docs>
         <param name="comment">A comment value.</param>
-        <summary>Initializes a new instance of the <see cref="T:System.Net.Http.Headers.ProductInfoHeaderValue" /> class.</summary>
-        <remarks>To be added.</remarks>
+        <summary>Initializes a new instance of the <see cref="T:System.Net.Http.Headers.ProductInfoHeaderValue" /> class with a comment.</summary>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+The syntax of a comment is defined in [RFC 2616](https://go.microsoft.com/fwlink/?LinkID=241642)
+  
+]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName=".ctor">
@@ -162,8 +169,15 @@
       <Docs>
         <param name="productName">The product name value.</param>
         <param name="productVersion">The product version value.</param>
-        <summary>Initializes a new instance of the <see cref="T:System.Net.Http.Headers.ProductInfoHeaderValue" /> class.</summary>
-        <remarks>To be added.</remarks>
+        <summary>Initializes a new instance of the <see cref="T:System.Net.Http.Headers.ProductInfoHeaderValue" /> class with product information.</summary>
+        <remarks>
+          <format type="text/markdown"><![CDATA[  
+  
+## Remarks  
+The syntax of product information (name and value) is defined in [RFC 2616](https://go.microsoft.com/fwlink/?LinkID=241642)
+  
+]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="Comment">