Span<T> indexer getter and setter, or only getter?

The documentation can't make up its mind whether the `Span` indexer only has a getter or both a getter and a setter.

It starts with "_Gets or sets the element at the specified zero-based index._", directly followed by showing the declaration having only a getter.

If the availability of the setter depends on certain platform versions, then this should be made clear in this documentation page.

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 7df2a61b-7d80-7dbd-6acd-213e3c80f715
* Version Independent ID: eb1b0d55-1d9b-1dcb-c51e-46d2f5937ea8
* Content: [Span<T>.Item\[Int32\] Property (System)](https://docs.microsoft.com/en-us/dotnet/api/system.span-1.item?view=netcore-2.2#System_Span_1_Item_System_Int32_)
* Content Source: [xml/System/Span`1.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System/Span`1.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**