You can't more often than not. Often the "why" is an implementation detail
regarding a single statement, and those shouldn't make their way to a function
summary.
If there's hidden meaning in an implementation detail regarding a single statement extract that value to one of the following:
a) a variable
b) a method
c) a class
All of these provide a mechanism for documenting the code without comments. Show me where you think this doesn't apply.