Merge pull request #16948 from bangerth/boundary-id

Make it clearer what an integer argument means.

diff --cc examples/step-3/doc/results.dox
Simple merge

diff --cc examples/step-3/step-3.cc
Simple merge

diff --cc examples/step-4/step-4.cc
Simple merge

diff --cc examples/step-5/step-5.cc
Simple merge

diff --cc examples/step-6/step-6.cc
Simple merge

diff --cc examples/step-7/step-7.cc
Simple merge

diff --cc examples/step-8/step-8.cc
Simple merge