Add doc for option all in add*set! (#527)

* Add doc for option all in add*set!

* Remove redundant default note

src/Grid/grid.jl

@@ -561,7 +561,9 @@ _warn_emptyset(set, name) = length(set) == 0 && @warn("no entities added to the
 
 Adds a cellset to the grid with key `name`.
 Cellsets are typically used to define subdomains of the problem, e.g. two materials in the computational domain.
-The `MixedDofHandler` can construct different fields which live not on the whole domain, but rather on a cellset. 
+The `MixedDofHandler` can construct different fields which live not on the whole domain, but rather on a cellset.
+`all=true` implies that `f(x)` must return `true` for all nodal coordinates `x` in the cell if the cell
+should be added to the set, otherwise it suffices that `f(x)` returns `true` for one node. 
 
 ```julia
 addcellset!(grid, "left", Set((1,3))) #add cells with id 1 and 3 to cellset left
@@ -600,6 +602,8 @@ end
 Adds a faceset to the grid with key `name`.
 A faceset maps a `String` key to a `Set` of tuples corresponding to `(global_cell_id, local_face_id)`.
 Facesets are used to initialize `Dirichlet` structs, that are needed to specify the boundary for the `ConstraintHandler`.
+`all=true` implies that `f(x)` must return `true` for all nodal coordinates `x` on the face if the face
+should be added to the set, otherwise it suffices that `f(x)` returns `true` for one node. 
 
 ```julia
 addfaceset!(gird, "right", Set(((2,2),(4,2))) #see grid manual example for reference