Is there a document that specifies un-installation of SQL server 2016 R server
Try this guide, maybe it will work for you: Cleanly Uninstalling Stubborn SQL Server Components by Aaron Bertrand.

The easiest way: Control Panel -> Programs &amp; Features -> and uninstall every program that contains the words "SQL Server". 

or, using PowerShell:

Get-ChildItem HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall | 
select @{Name='Guid';Expression={$_.PSChildName}}, @{Name='Disp';Expression={($_.GetValue("DisplayName"))}} | 
where-object {$_.Disp -ilike "*SQL*R"} | 
where-object {$_.Guid -like '{*'} | 
% {"rem " + $_.Disp; 'msiexec /x "' + $_.Guid + '"'; ''} &gt; uninstallR2.bat