Correct doc on ReadOnlySequence<T>.IsSingleSegment (#11269)

The current property value was presumably intended for IsEmpty

Author: Rob-Hague

xml/System.Buffers/ReadOnlySequence`1.xml

@@ -580,7 +580,7 @@
       </ReturnValue>
       <Docs>
         <summary>Gets a value that indicates whether the <see cref="T:System.Buffers.ReadOnlySequence`1" /> is empty.</summary>
-        <value>To be added.</value>
+        <value><see langword="true" /> if the read-only sequence is empty; otherwise, <see langword="false" />.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -615,8 +615,7 @@
       </ReturnValue>
       <Docs>
         <summary>Gets a value that indicates whether the <see cref="T:System.Buffers.ReadOnlySequence`1" /> contains a single <see cref="T:System.ReadOnlyMemory`1" /> segment.</summary>
-        <value>
-          <see langword="true" /> if the read-only sequence is empty; otherwise, <see langword="false" />.</value>
+        <value><see langword="true" /> if the read-only sequence consists of a single memory segment; otherwise, <see langword="false" />.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>