How to add value to description property of a powershell function?

You should use comment based help instead of usage functions. Comment based help will allow you to work with the build in help system of PowerShell. It isn't hard to write and you don't even need to use all the sections.

Here is the TechNet on about_Comment_Based_Help, with some examples:

The syntax for comment-based Help is as follows:

# .< help keyword>
# <help content>

-or -

<#
    .< help keyword>
    < help content>
#>

You do have to make sure the categories are spelled correctly, otherwise the whole help on your function won't show up and you won't get an error message. A simple example that just has a synopsis and a description:

 .SYNOPSIS
      A brief description of the function or script. This keyword can be used
      only once in each topic.

 .DESCRIPTION
      A detailed description of the function or script. This keyword can be
      used only once in each topic.

Read the linked article for all the keywords and their descriptions. Similar information can be found by running get-help about_comment_based_help

So far the responses do not answer the actual question as asked, and it is a good question as is, even though adding Help is a good and useful idea, whether it is Comment Based or full XML Help.

The problem is that Comment Based Help only provides the "Description" for the functions HELP, and not it's properties as shown when you run Get-Command.

Getting the Description with Get-Command is a very useful and separate idea.

However, only some properties of a function are (directly) settable and we can discover this with Get-Member -membertype Property as shown in the original question and whose output includes (part of) the answer:

Description         Property       string Description {get;set;}

Member property 'Description' is directly SETTABLE, as shown as the end of the definition for the property: {get;set;}:

ANSWER: So we can easily set the functions actual Definition property with a direct assignment:*

   (Get-Command -Type Function -Name Get-AllColors).Description = 'Defined in $Profile'
# or
    (Get-Command -Type Function -Name Get-AllColors).Description = "Defined in the $Profile"

We can find all properties which are directly settable quite easily.

gcm mypshost | get-member | findstr "set;"
#  Typically only shows 3 properties unless the function was defined in a module
Description
Options
Visibility

Or perhaps even better would be setting the "Source" property which is now included in the default output from Get-Command (though that is not so trivially set with direct assignment.)

Creating the function within a Module with a proper "Module Manifest" does allow for setting most/all of these properties that are typically blank on functions defined directly.

The Source will be the "module" that defines the function and the version will be set as well if it is set in the module.

Unfortunately, the "Set-ItemProperty" command is not supported by the "Function" PSProvider (as it is by the Registry Provider and many others. The following fails (on Version 2.0 and 5.1):

Set-ItemProperty function:mypshost -Name Description -Value 'only testing'

While it works with similar syntax for a file:

Set-ItemProperty (dir ic.ps1) -Name LastWriteTime -Value (get-date)

Another example is the "OutputType" which can be set by using one of the options in the [CmdletBinding()] attribute of an advanced function parameter block:

I think what HerbM is looking for is a quick reminder what a function does, not a replacement for help. There is a good case for both. For example,I have a function that displays a list of commands and then executes them. If I enter:

EXAMPLE

gcm na|select -property name,description,DisplayName
Name Description                        DisplayName
---- -----------                        -----------
NA   RB Do a command with no arguments. NA -> Do_Noarg

GCM lists a short description that I included when I created the alias NA.

It would be useful if GCM would also list the description on functions instead of blanks as follows:

EXAMPLE

gcm do_noarg|select -property name,description,Definition

Name     Description Definition
----     ----------- ----------
Do_Noarg              dr      ~\PS1\No_Arg\*.ps1 Invoke-Expression

I believe HerbM isn’t looking for gcm or any other tool to replace help but Just wants a quick single line description of a function. If Description is just a placeholder for future releases of “function”, then please let everyone know.

You can use Comment-Based Help to add a description (and lots of other information) to your functions:

 SYNTAX FOR COMMENT-BASED HELP
    The syntax for comment-based help is as follows:

        # .< help keyword>
        # <help content>

    -or -

        <#
            .< help keyword>
            < help content>
        #>

Example:

function Add-Extension 
{
        param ([string]$Name,[string]$Extension = "txt")
        $name = $name + "." + $extension
        $name

    <#
    .SYNOPSIS 
    Adds a file name extension to a supplied name.

    .DESCRIPTION
    Adds a file name extension to a supplied name. 
    Takes any strings for the file name or extension.
    }