Automatically port System.Text.Json triple slash comments

[carlossanlop]

I synced my dotnet-api-docs clone to the latest bits to ensure I had the latest xml API structure from the most recent reference assembly update. I also synced my corefx repo to the latest bits and built them to ensure I had the latest comments.

Area owners @ahsonkhan @steveharter please help double check that the descriptions are all accurate.
@mairaw @rpetrusha for language review.

[carlossanlop]

@rpetrusha do you mind checking this PR next?

[carlossanlop]

@ahsonkhan @steveharter please help review these ported comments.

[rpetrusha]

I'm checking it now, @carlossanlop.

[rpetrusha]

I've started making changes up to Utf8JsonReader, @carlossanlop. I'll finish tomorrow. Please review what I've finished so far. Tomorrow, I'll finish and approve, you can review, and I'll merge.

[carlossanlop]

@ahsonkhan there are multiple complex requests that should be addressed separately due to recent changes in source. Let's get this PR merged first, then update the documentation in a new PR.

@rpetrusha @mairaw I addressed the last comments. Can you please take a last look to see if we can get it merged?

[rpetrusha]

This looks good, @carlossanlop, except that the build report showed 7 broken crefs. I think I've fixed them all, so I'll merge when a successful build completes.

[carlossanlop]

@thanks @rpetrusha. I fixed additional JSON literals that needed to be changed from "word" to <b>word</b>

[ahsonkhan]

A bunch of APIs changed between preview6/preview7. @rpetrusha - thanks for fixing some of the links.
Are you confident we reacted to all of them?
https://github.com/dotnet/core/blob/master/release-notes/3.0/preview/api-diff/preview7/3.0-preview7_System.Text.Json.md
https://github.com/dotnet/core/blob/master/release-notes/3.0/preview/api-diff/preview7/3.0-preview7_System.Text.Json.Serialization.md

[rpetrusha]

I think so, @ahsonkhan. We've updated the API reference to reflect Preview 7 APIs, and the build report isn't showing any additional broken links.

[ahsonkhan]

Looks like we are missing the docs from source for the the other property, ValueKind:

dotnet-api-docs/xml/System.Text.Json/JsonElement.xml

Lines 1527 to 1547 in c7324df

	<Member MemberName="ValueKind">
	<MemberSignature Language="C#" Value="public System.Text.Json.JsonValueKind ValueKind { get; }" />
	<MemberSignature Language="ILAsm" Value=".property instance valuetype System.Text.Json.JsonValueKind ValueKind" />
	<MemberSignature Language="DocId" Value="P:System.Text.Json.JsonElement.ValueKind" />
	<MemberSignature Language="VB.NET" Value="Public ReadOnly Property ValueKind As JsonValueKind" />
	<MemberSignature Language="C++ CLI" Value="public:&#xA; property System::Text::Json::JsonValueKind ValueKind { System::Text::Json::JsonValueKind get(); };" />
	<MemberSignature Language="F#" Value="member this.ValueKind : System.Text.Json.JsonValueKind" Usage="System.Text.Json.JsonElement.ValueKind" />
	<MemberType>Property</MemberType>
	<AssemblyInfo>
	<AssemblyName>System.Text.Json</AssemblyName>
	<AssemblyVersion>4.0.0.0</AssemblyVersion>
	</AssemblyInfo>
	<ReturnValue>
	<ReturnType>System.Text.Json.JsonValueKind</ReturnType>
	</ReturnValue>
	<Docs>
	<summary>To be added.</summary>
	<value>To be added.</value>
	<remarks>To be added.</remarks>
	</Docs>
	</Member>

https://github.com/dotnet/corefx/blob/9e994a5923d426f0e64ed3b8f9b93df6f214a42a/src/System.Text.Json/src/System/Text/Json/Document/JsonElement.cs#L32-L38

[ahsonkhan]

We are missing parameter description of this indexer:
https://docs.microsoft.com/en-us/dotnet/api/system.text.json.jsonelement.item?view=netcore-3.0#System_Text_Json_JsonElement_Item_System_Int32_

dotnet-api-docs/xml/System.Text.Json/JsonElement.xml

Lines 787 to 795 in c7324df

	<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>
	<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>
	<exception cref="T:System.ObjectDisposedException">The parent <see cref="T:System.Text.Json.JsonDocument" /> has been disposed.</exception>
	</Docs>

It doesn't exist in source either.

@bartonjs, can you please wordsmith something here to describe the JsonValueKind.Array indexer?
https://github.com/dotnet/corefx/blob/9e994a5923d426f0e64ed3b8f9b93df6f214a42a/src/System.Text.Json/src/System/Text/Json/Document/JsonElement.cs#L40-L61