add comments about sed commands

ensure-one-newline()

{

    # first command adds a newline at the end of the file if there might be none

    # -> clang-format-13 does not add them and warnings e.g. on MacOS are triggered by them

    sed -i -e '$a\' $1

    # second command removes multiple newlines at the end of a file so that there is only one left

    # -> clang-format-13 does not do that, but complains about it

    sed -i -e :a -e '/^\n*$/{$d;N;ba' -e '}' $1

}