Add remaining doc from issue 105988 (#10259)

Author: buyaa-n

xml/System.Runtime.CompilerServices/ParamCollectionAttribute.xml

@@ -37,7 +37,7 @@
       </AssemblyInfo>
       <Parameters />
       <Docs>
-        <summary>To be added.</summary>
+        <summary>Initializes a new instance of the <see cref="T:System.Runtime.CompilerServices.ParamCollectionAttribute" />class.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.Text/StringBuilder.xml

@@ -7035,7 +7035,7 @@ In .NET Core and in the .NET Framework 4.0 and later versions, when you instanti
         <param name="oldValue">The read-only character span to replace.</param>
         <param name="newValue">The read-only character span to replace <paramref name="oldValue" /> with.</param>
         <summary>Replaces all instances of one read-only character span with another in this builder.</summary>
-        <returns>To be added.</returns>
+        <returns>A reference to this instance with <paramref name="oldChar" /> replaced by <paramref name="newChar" />.</returns>
         <remarks>If <paramref name="newValue" /> is empty, instances of <paramref name="oldValue" /> are removed from this builder.</remarks>
       </Docs>
     </Member>
@@ -7233,7 +7233,7 @@ In .NET Core and in the .NET Framework 4.0 and later versions, when you instanti
         <param name="startIndex">The index to start in this builder.</param>
         <param name="count">The number of characters to read in this builder.</param>
         <summary>Replaces all instances of one read-only character span with another in part of this builder.</summary>
-        <returns>To be added.</returns>
+        <returns>A reference to this instance with <paramref name="oldChar" /> replaced by <paramref name="newChar" />.</returns>
         <remarks>If <paramref name="newValue" /> is empty, instances of <paramref name="oldValue" /> are removed from this builder.</remarks>
       </Docs>
     </Member>