Document System.StringComparer (#3648)

* Document System.StringComparer.FromComparison

* Document Create

Author: carlossanlop

xml/System/StringComparer.xml

@@ -367,11 +367,15 @@ The following example demonstrates the properties and the <xref:System.StringCom
         <Parameter Name="options" Type="System.Globalization.CompareOptions" Index="1" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="culture">To be added.</param>
-        <param name="options">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="culture">A culture whose linguistic rules are used to perform a string comparison.</param>
+        <param name="options">A bitwise combination of the <see cref="T:System.Globalization.CompareOptions" /> values.</param>
+        <summary>Creates a <see cref="T:System.StringComparer" /> object that compares strings according to the rules of a specified culture and string options.</summary>
+        <returns>A new <see cref="T:System.StringComparer" /> object that performs string comparisons according to the comparison rules used by the <paramref name="culture" /> parameter and the specified <paramref name="options" />.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentNullException">
+          <paramref name="culture" /> is <see langword="null" />.</exception>
+        <exception cref="T:System.ArgumentException">
+          <paramref name="options" /> has invalid flags.</exception>
       </Docs>
     </Member>
     <Member MemberName="CurrentCulture">
@@ -637,10 +641,11 @@ The following example demonstrates the properties and the <xref:System.StringCom
         <Parameter Name="comparisonType" Type="System.StringComparison" Index="0" FrameworkAlternate="netcore-2.0;netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="comparisonType">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="comparisonType">A string comparer instance to convert.</param>
+        <summary>Converts the specified <see cref="T:System.StringComparison" /> instance to a <see cref="T:System.StringComparer" /> instance.</summary>
+        <returns>A <see cref="T:System.StringComparer" /> instance representing the equivalent value of the specified <see cref="T:System.StringComparison" /> instance.</returns>
         <remarks>To be added.</remarks>
+        <exception cref="T:System.ArgumentException">The specified string comparison is not supported.</exception>
       </Docs>
     </Member>
     <MemberGroup MemberName="GetHashCode">