Make LeadingMarginSpan2 behavior more consistent

This addresses b/16486549.

This change updates public documentation to specify the behavior of
LeadingMarginSpan2s. This change specifies what happens when a
LeadingMarginSpan2 is combined with other LeadingMarginSpans. This
behavior was not previously documented.

LeadingMarginSpan2s specify the number of lines used for the leading
margin. When laying out and rendering, for all LeadingMarginSpans, the
first line margin is applied for the number of lines specified by the
LeadingMarginSpan2.

Previously, this behavior was slightly buggy -- the LeadingMarginSpan2
affected all LeadingMarginSpans when laying out text, but not when
rendering.

This change is designed to cause the least amount of breakage in
existing code while achieving consistency with the way
LeadingMarginSpan2 is handled in layout and drawing.

For the most common use of LeadingMarginSpan2 -- getting a multi-line
first margin in the first paragraph of text in a layout -- this should
cause no change in behavior. For any other uses, the old (buggy)
implementation most likely did not exhibit correct behavior to begin
with, so developers were most likely not relying on that functionality.

Change-Id: I6f69df09c0130e703458e65bf3eaac4a905df56e