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.
+ When the optional base level parameter is provided, we no
longer ensure that the sequence of heading levels is gapless
[behavior change]. Instead, we set the lowest heading level to
the specified base level, and adjust the others accordingly. If
an author wants to skip a level, e.g. from level 1 to level 3,
they can do that. In general, the heading levels specified
in the source document are preserved; `makeSections` only
puts them into a hierarchical structure. Closes#9398.
+ Section numbers are now assigned so that the top level
gets `1`, no matter what heading level is used. So, even
if the top heading level is 2, numbers will be `1`, `2`, etc.
rather than `0.1`, `0.2`, as in the past. Closes#5071.
+ We revert to the old behavior when the `--number-offset` option
is used. So, for example, if a document begins with a level-3
heading, and `--number-offset=1,2` is used, the top-level section
numbers will be `1.2.1`, `1.2.2`, etc. This is mainly for
backwards-compatibility.
John MacFarlane