Do not strip leading whitespace when parsing doc comments.

This change prevents the indentation in code blocks inside the /// doc comments
from being eaten. The indentation that is the same across the consecutive doc
comments is removed by the uindent_pass in librustdoc.

The bug can be seen, e.g., here: http://static.rust-lang.org/doc/std/iterator.html#example-12

I also altered how the block comments are treated, for consistency. There isn't much testing done on the documentation output (I added a few tests of my own for the modified function), so I don't know if anything relied on the previous behavior. I checked a number of documentation files and observed either no change in output or changes that consistent of the above bug being fixed.