kernel-doc: allow multi-line declaration purpose descriptions

Allow the short description after symbol name and dash in a kernel-doc
comment to span multiple lines, e.g. like this:

	/**
	 * unmap_mapping_range - unmap the portion of all mmaps in the
	 *	specified address_space corresponding to the specified
	 *	page range in the underlying file.
	 * @mapping: the address space containing mmaps to be unmapped.
	 * ...
	 */

The short description ends with a parameter description, an empty line
or the end of the comment block.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>