What is included in ALL permissions for functions in PostgreSQL?
The only permission applicable to functions is EXECUTE, so when you use ALL postgresql will try to apply all the permissions available for functions, and for now this is just "EXECUTE". 

So it means exactly the same. 


  EXECUTE
  
  Allows the use of the specified function and the use of any operators
  that are implemented on top of the function. This is the only type of
  privilege that is applicable to functions. 


https://www.postgresql.org/docs/10/static/sql-grant.html

But be carefully if more kind of permissions for functions be implemented in new postgresql versions this can do wrong (but i can't see other than execute).