Add documentation for new ForEach-Object -Parallel parameter set

[PaulHigin]

Issue Details

This is associated with RFC: PowerShell/PowerShell-RFC#194

Version(s) of document impacted

• Impacts 7 document
• Impacts 6 document
• Impacts 5.1 document
• Impacts 5.0 document
• Impacts 4.0 document
• Impacts 3.0 document

Reason(s) for not selecting all version of documents

• The documented feature was introduced in selected version of PowerShell
• This issue only shows up in selected version of the document

[iSazonov]

From Paul Higinbotham (@PaulHigin)

I'll work on creating a blog that I hope will help make clear when using foreach -parallel is beneficial.

It would be nice to have this in documentation too.

[PaulHigin]

Document has been updated with PR: #4796