Update about_Properties to introduce the term member enumeration and to document the behavior with array-valued properties #2198

PowerShell/PowerShell#6454 discusses the perhaps surprising behavior of member enumeration with array-valued properties.

The current about_Properties help topic:

• discusses member enumeration - a term coined in this 2012 PowerShell team blog post - only in the abstract and never gives it a name.

• unlike the blog post, does not discuss the implications of using member enumeration with array-valued properties; in short:

  • ( @{ a=1,2 }, @{ a=3,4 }).a is the equivalent of @{ a=1,2 }, @{ a=3,4 } | ForEach-Object { $_.a } (resulting in 1, 2, 3, 4, a flat array of 4 elements) and not, as is another possible interpretation of the current documentation, (1, 2), (3, 4) (an array with 2 elements whose elements are 2-element arrays).

Version(s) of document impacted

• Impacts 6 document
• Impacts 5.1 document
• Impacts 5.0 document
• Impacts 4.0 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