Fix wrong docs: CellIterator, assemble!

[KnutAM]

grid=generate_grid(Quadrilateral, (20, 20));
dh=DofHandler(grid); push!(dh, :u, 2); close!(dh);
CellIterator(dh).current_cellid[]

gives 0
(Fixing my own mistakes, ref #414 )

Edit: Added another doc error I discovered for assemble!(g, edof, ge) and assemble!(K, edof, Ke)
I also suggest renaming edof here to dofs to be consistent with the other assemble! methods

[codecov-commenter]

Codecov Report

Merging #467 (34e2078) into master (a6a2503) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #467   +/-   ##
=======================================
  Coverage   91.34%   91.34%           
=======================================
  Files          22       22           
  Lines        3407     3407           
=======================================
  Hits         3112     3112           
  Misses        295      295           

Impacted Files	Coverage Δ
src/iterators.jl	96.92% <ø> (ø)
src/assembler.jl	98.34% <100.00%> (ø)

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