add some docstrings (#414)

docs/src/reference/dofhandler.md

@@ -6,4 +6,7 @@ DocTestSetup = :(using Ferrite)
 
 ```@docs
 DofHandler
+CellIterator
+ndofs
+dof_range
 ```

src/Dofs/DofHandler.jl

@@ -44,6 +44,11 @@ function Base.show(io::IO, ::MIME"text/plain", dh::DofHandler)
     end
 end
 
+"""
+    ndofs(dh::AbstractDofHandler)
+
+Return the number of degrees of freedom in `dh`
+"""
 ndofs(dh::AbstractDofHandler) = dh.ndofs[]
 ndofs_per_cell(dh::AbstractDofHandler, cell::Int=1) = dh.cell_dofs_offset[cell+1] - dh.cell_dofs_offset[cell]
 isclosed(dh::AbstractDofHandler) = dh.closed[]
@@ -81,7 +86,8 @@ end
 Return the local dof range for `field_name`. Example:
 
 ```jldoctest
-julia> grid = generate_grid(Triangle, (3, 3));
+julia> grid = generate_grid(Triangle, (3, 3))
+Grid{2, Triangle, Float64} with 18 Triangle cells and 16 nodes
 
 julia> dh = DofHandler(grid); push!(dh, :u, 3); push!(dh, :p, 1); close!(dh);
 

src/iterators.jl

@@ -10,18 +10,26 @@ UpdateFlags(; nodes::Bool=true, coords::Bool=true, celldofs::Bool=true) =
 
 """
     CellIterator(grid::Grid)
-    CellIterator(grid::DofHandler)
+    CellIterator(dh::DofHandler)
+    CellIterator(mdh::MixedDofHandler)
 
 Return a `CellIterator` to conveniently loop over all the cells in a grid.
 
 # Examples
 ```julia
-for cell in CellIterator(grid)
+for cell in CellIterator(dh)      # dh::DofHandler
     coords = getcoordinates(cell) # get the coordinates
     dofs = celldofs(cell)         # get the dofs for this cell
     reinit!(cv, cell)             # reinit! the FE-base with a CellIterator
 end
 ```
+Here, `cell::CellIterator`. Looking at a specific cell (instead of 
+looping over all), e.g. nr 10, can be done by
+```julia
+cell = CellIterator(dh)     # Refers to cell nr. 1 upon creation
+reinit!(cell, 10)           # Update to cell nr. 10
+dofs = celldofs(cell)       # Get the dofs for cell nr. 10
+```
 """
 struct CellIterator{dim,C,T,DH<:Union{AbstractDofHandler,Nothing}}
     flags::UpdateFlags