The section on pseudo-types names "void" and describes is thusly:
"""
Indicates that a function returns no value.
"""
This can be reasonably interpreted to mean: "Use this when you don't want the execution of a function to return a value." (i.e., the empty set)
The following paragraph, suitably marked up, would be a nice addition:
If a function declares "RETURNS void" the result of executing the function will be a single row and column whose type pg_typeof() returns "void". Void is not null and typically displays textually as the empty string.
I'd maybe modify the table to read:
void : A value that has no content: used to indicate that a function returns no value.
David J.