Document various isolated System.Text.Json elements (#2910)

* Document various isolated System.Text.Json elements

* Update JsonStringEnumConverter.xml

* Update JsonElement.xml

Author: carlossanlop

xml/System.Text.Json.Serialization/JsonStringEnumConverter.xml

@@ -84,9 +84,9 @@ Reading is case insensitive. Writing can be customized by using a <xref:System.T
         <Parameter Name="typeToConvert" Type="System.Type" />
       </Parameters>
       <Docs>
-        <param name="typeToConvert">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="typeToConvert">The type to be checked.</param>
+        <summary>Determines whether the specified type can be converted to an enum.</summary>
+        <returns><see langword="true" /> if the type can be converted; otherwise, <see langword="false" />.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -109,9 +109,9 @@ Reading is case insensitive. Writing can be customized by using a <xref:System.T
         <Parameter Name="typeToConvert" Type="System.Type" />
       </Parameters>
       <Docs>
-        <param name="typeToConvert">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="typeToConvert">The type to convert.</param>
+        <summary>Create a converter for the specified type.</summary>
+        <returns>An instance of <see cref="T:System.Text.Json.Serialization.JsonConverter"/> of type T, where T is compatible with <paramref name="typeToConvert"/>.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.Text.Json/JsonElement.xml

@@ -837,9 +837,9 @@ This method does not parse the contents of a JSON string value.
         <Parameter Name="index" Type="System.Int32" />
       </Parameters>
       <Docs>
-        <param name="index">To be added.</param>
-        <summary>Gets the value at a specified index when the current value is an <see cref="F:System.Text.Json.JsonValueKind.Array" />.</summary>
-        <value>To be added.</value>
+        <param name="index">The item index.</param>
+        <summary>Gets the value at the specified index if the current value is an <see cref="F:System.Text.Json.JsonValueKind.Array" />.</summary>
+        <value>The value at the specified index.</value>
         <remarks>To be added.</remarks>
         <exception cref="T:System.InvalidOperationException">This value's <see cref="P:System.Text.Json.JsonElement.ValueKind" /> is not <see cref="F:System.Text.Json.JsonValueKind.Array" />.</exception>
         <exception cref="T:System.IndexOutOfRangeException"><paramref name="index" /> is not in the range [0, <see cref="M:System.Text.Json.JsonElement.GetArrayLength" />()).</exception>

xml/System.Text.Json/JsonEncodedText.xml

@@ -108,7 +108,7 @@ This type can be used to cache and store known strings used for writing JSON ahe
       <Docs>
         <param name="value">The value to be transformed as JSON encoded text.</param>
         <summary>Encodes the string text value as a JSON string.</summary>
-        <returns>To be added.</returns>
+        <returns>The string encoded as a JSON string.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentNullException"><paramref name="value" /> is <see langword="null" />.</exception>
         <exception cref="T:System.ArgumentException"><paramref name="value" /> is too large or contains invalid UTF-16 characters.</exception>
@@ -154,7 +154,7 @@ This type can be used to cache and store known strings used for writing JSON ahe
         <Parameter Name="obj" Type="System.Object" />
       </Parameters>
       <Docs>
-        <param name="obj">To be added.</param>
+        <param name="obj">The object to compare to this instance.</param>
         <summary>Determines whether this instance and a specified object, which must also be a <see cref="T:System.Text.Json.JsonEncodedText" /> instance, have the same value.</summary>
         <returns><see langword="true" /> if the current instance and <paramref name="obj" /> are equal; otherwise, <see langword="false" />.</returns>
         <remarks>
@@ -190,7 +190,7 @@ If `obj` is `null`, the method returns `false`.
         <Parameter Name="other" Type="System.Text.Json.JsonEncodedText" />
       </Parameters>
       <Docs>
-        <param name="other">To be added.</param>
+        <param name="other">The object to compare to this instance.</param>
         <summary>Determines whether this instance and another specified <see cref="T:System.Text.Json.JsonEncodedText" /> instance have the same value.</summary>
         <returns><see langword="true" /> if this instance and <paramref name="other" /> have the same value; otherwise, <see langword="false" />.</returns>
         <remarks>