This is like `makeSections` but has an additional parameter
specifying number offsets, for use with the `--number-offset` option.
Use `makeSectionsWithOffsets` in HTML writer instead of ad hoc
and inefficient number-adjusting code.
Clarify MANUAL.txt: the `--number-offset` option should only
directly affect numbering of the first section heading in
a document; subsequent headings will increment normally.
Fix test output for #5071 to reflect this.
John MacFarlane
3e6eb1cde2
