rustdoc: short descr. cause word-breaks in tables

The `.item-table` class is used to display name+description lists, e.g.
the exported functions, as a table. If the names are long and the
descriptions are short, then the width of the table does not expand to
the whole size, but only uses a fraction. This causes a some names to
break inside a word.

This change makes the table always use 100% of its parent width. The
`.width-limiter` wrapper already ensures that the used width still does
not become excessive.

Signed-off-by: René Kijewski <rene.kijewski@fu-berlin.de>