Commit 6f6257476754 ("Convert documentation to reStructuredText")
automatically converted all documents by a tool.  I see some parts
were converted in an ugly way (or, at least, it is not my intention).
Also, the footnote is apparently broken.

I checked this document by my eyes, and reformated it so that it looks
nicer both in plain text and reST form.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>