With a Select List custom field, it is not presently possible to add a Description for each option. Once implemented this benefits the user in various ways:

(1) Shown inside the combo dropdown:
[Critical - Crashes, exceptions, can't access feature]
[Major - loss of function with no workaround ]
Where the option is in bold and the description in light text is shown after it.

(2) Hover text for the currently selected option can be displayed when hovering over the Field.

(3) Automatically generating a yellow Help icon that could be placed to the right of a custom field - same as Priority, etc - that shows all these Options and their Descriptions.

Currently, the only way to educate users on the meaning of custom field options is to describe them all in the Description - this produces a very long block of text that takes up space on the page and annoys the more experienced users.