block: Document genhd capability flags

The kernel documentation includes a brief section about genhd
capabilities, but it turns out that the only documented
capability (GENHD_FL_MEDIA_CHANGE_NOTIFY) isn't used any more.

This patch removes that flag, and documents the rest, based on my
understanding of the current uses of these flags in the kernel. The
documentation is kept in the header file, alongside the declarations,
in the hope that it will be kept up-to-date in future; the kernel
documentation is changed to include the documentation generated from
the header file.

Because the ultimate goal is to provide some end-user
documentation (or end-administrator documentation), the comments are
perhaps more user-oriented than might be expected. Since the values
are shown to users in hexadecimal, the documentation lists them in
hexadecimal, and the constant declarations are adjusted to match.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>