Out of the box functionality, you can type in comments through PA30 on some of the infotypes.
But how do you have this cool feature for other infotypes, such as custom infotypes or infotypes not automatically default out of the box?
You can! Very easy as the matter of fact. You can point your table to V_T582A or go to the IMG via SPRO -> Personnel Management -> Personnel Administration -> Customizing Procedures -> Infotypes -> Infotypes
Select the infotype you want to maintain and select "Text Allowed". Whoa la!