Add note about need for experimental feature

[sdwheeler]

Prerequisites

• Write a descriptive title.
• Search the existing issues.
• I am reporting the documentation problem for version of PowerShell I am using.

Version

7.2

Link to affected document

https://docs.microsoft.com/en-us/powershell/scripting/dev-cross-plat/create-cmdline-predictor

Description of the documentation error

Using Get-PSSubsystem requires an experimental feature.

Suggested fix

Add a note to the article about the requirement.