Problem:
In the generated docs HTML there is too much whitespace before/after `<pre>`
blocks.
- In the old layout (fixed-width), all text in `.old-help-para` is formatted as
`white-space:pre`.
- In the new layout, when `<pre>` is at the end of a `<div>`, the margins of
both are redundant, causing too much space.
Solution:
- In the old layout, always remove `<pre>` margin.
- In the new layout, disable `<pre>` margin if it is the last child.
957d05d16b
