Add remianing missing doc for Base64Url (#10248)

* Add remianing missing doc for Base64Url

* Update xml/System.Buffers.Text/Base64Url.xml

Co-authored-by: Genevieve Warren <[email protected]>

---------

Co-authored-by: Genevieve Warren <[email protected]>

Author: buyaa-n

xml/System.Buffers.Text/Base64Url.xml

@@ -14,7 +14,7 @@
   </Base>
   <Interfaces />
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Converts between binary data and URL-safe ASCII encoded text that's represented in Base64Url characters.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>
@@ -525,9 +525,9 @@
         <Parameter Name="bytesLength" Type="System.Int32" />
       </Parameters>
       <Docs>
-        <param name="bytesLength">To be added.</param>
+        <param name="bytesLength">The size of the input span.</param>
         <summary>Returns the length (in bytes) of the result if you were to encode binary data within a byte span of size <paramref name="bytesLength" />.</summary>
-        <returns>To be added.</returns>
+        <returns>The length (in bytes) of the encoded result.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">
           <paramref name="bytesLength" /> is less than 0 or greater than 1610612733.</exception>
@@ -552,9 +552,9 @@
         <Parameter Name="base64Length" Type="System.Int32" />
       </Parameters>
       <Docs>
-        <param name="base64Length">To be added.</param>
+        <param name="base64Length">The size of the input span.</param>
         <summary>Returns the maximum length (in bytes) of the result if you were to decode base 64 encoded text from a span of size <paramref name="base64Length" />.</summary>
-        <returns>To be added.</returns>
+        <returns>The maximum length (in bytes) of the decoded result.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">The specified <paramref name="base64Length" /> is less than 0.</exception>
       </Docs>