Remove onboundary from examples

[KnutAM]

If removed from Neumann example, probably good to remove from examples as well? Ref #473

Edit: Also adopted an unrelated doc change from @fredrikekre's comment in #470

[codecov-commenter]

Codecov Report

Merging #474 (c60b89e) into master (b9998a0) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
- Coverage   91.35%   91.32%   -0.03%     
==========================================
  Files          22       22              
  Lines        3411     3411              
==========================================
- Hits         3116     3115       -1     
- Misses        295      296       +1     

Impacted Files	Coverage Δ
src/iterators.jl	95.38% <0.00%> (-1.54%)	⬇️

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[lijas]

Move the face integration outside the cell loop and do:

for (cellid, faceid) in union(getfaceset(grid, "left"), getfaceset(grid, "right") )

is my prefered way :)

[KnutAM]

I like that better too, and it is what would be done by the BoundaryFaceIterator that is under construction!

[KnutAM]

Handled in #495