Mapping Center Answer:

Unfortunately, at this time, there is no out of the box automated way to do what you are asking.

You might want to look through the ArcScripts website to see if someone has written a script that automates this process.

For the manual method do the following:

• Right click on your feature layer in the Table of Contents and go to its Properties
• On the Symbology tab, right click on one of the patch shapes and select "Edit Description..."
• Once the Description for Legend dialog comes up, copy and paste the detailed description from your table here for the symbol that you selected
• If you want your description to be more than one line, press CTRL+ENTER to insert a line break
• If you want to further format the look of the text, you can use text formatting tags

What this will give you in your legend is the abbreviated label and next to it, the longer, more descriptive text. 

There is a sample graphic here that is similar to how your legend will look.

Note that you can also select from existing legend styles to design the layout of your legend.  Right click the legend on the page layout and click Properties.  Then on the Items tab, click the Styles button to see if there is a legend style that will work for your layout.

Another Mapping Center blog post that you might find useful is Organizing layers with too many unique values.