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>
diff --git a/Documentation/kernel-doc-nano-HOWTO.txt b/Documentation/kernel-doc-nano-HOWTO.txt
index 4d04572..348b9e5 100644
--- a/Documentation/kernel-doc-nano-HOWTO.txt
+++ b/Documentation/kernel-doc-nano-HOWTO.txt
@@ -66,7 +66,9 @@
  * The longer description can have multiple paragraphs.
  */
 
-The first line, with the short description, must be on a single line.
+The short description following the subject can span multiple lines
+and ends with an @argument description, an empty line or the end of
+the comment block.
 
 The @argument descriptions must begin on the very next line following
 this opening short function description line, with no intervening