Parameter help is most often written using the .PARAMETER tag, as shown in the following example:

function Get-Something {
    <#
    .SYNOPSIS
        Briefly describes the main action performed by Get-Something
    .DESCRIPTION
        A detailed description of the activities of Get-Something.
    .PARAMETER Parameter1
        Describes the purpose of Parameter1.
    .PARAMETER Parameter2
        Describes the purpose of Parameter2.
    #>

    param (
        $Parameter1,
        $Parameter2
    )
}

It is also possible to write the help for a parameter above the parameter itself:

function Get-Something {
    <#
    .SYNOPSIS
        Briefly describes the main action performed by Get-Something
    .DESCRIPTION
        A detailed description of the activities of Get-Something.
    #>

    param (
        # Describes the purpose of Parameter1.
        $Parameter1,

        # Describes the purpose of Parameter2.
        $Parameter2
    )
}

One possible advantage of this approach is that it is easy to see which parameters have help and which do not.

Regardless of where help is written for a parameter, Get-Help will read the value:

PS> Get-Help Get-Something -Parameter Parameter1

-Parameter1 <Object>
    Describes the purpose of Parameter1.

    Required? false
    Position? 1
    Default value
    Accept pipeline input? false
    Accept wildcard characters? false