Fixes #11955 - Add notes about the __AllParameterSets name

[sdwheeler]

PR Summary

Add notes about the __AllParameterSets name

• Fixes AB#410960
• Fixes Doc __AllParameterSets in about_Parameter_Sets? #11955

PR Checklist

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• Summary: This PR's summary describes the scope and intent of the change.
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reference/7.4/Microsoft.PowerShell.Core/About/about_Parameter_Sets.md	✅Succeeded	View (powershell-7.4)
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Learn Build status updates of commit 6d9a0b6:

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reference/5.1/Microsoft.PowerShell.Core/About/about_Parameter_Sets.md	✅Succeeded	View (powershell-5.1)
reference/7.4/Microsoft.PowerShell.Core/About/about_Parameter_Sets.md	✅Succeeded	View (powershell-7.4)
reference/7.5/Microsoft.PowerShell.Core/About/about_Parameter_Sets.md	✅Succeeded	View (powershell-7.5)
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[indented-automation]

Flagging and will try and find a moment to edit. But this assertion is incorrect (came up in the / a PS discord):

- Setting **DefaultParameterSetName** to `__AllParameterSets` in the
  `[CmdletBinding()]` attribute has no effect.

It will cause creation of an explicit parameter set if the value is set to __AllParameterSets, but the result will be quite confusing (for a named set).

It would have been nice if PS would outright reject the value, but since it doesn't it perhaps just needs a "__AllParameterSets should not be explicitly used as the DefaultParameterSetName" or similar.

This will have two parameter sets:

function a { [CmdletBinding(DefaultParameterSetName = '__AllParameterSets')]param ( [Parameter(ParameterSetName = 'NotDefault')]$a ) }
gcm a -synt

This only has one:

function a { [CmdletBinding()]param ( [Parameter(ParameterSetName = 'NotDefault')]$a ) }
gcm a -synt

Tested in 5.1 and 7.5, but if it differs anywhere I'd be very surprised.