CodingStyle: Document the exception of not splitting user-visible strings, for grepping

CodingStyle: Document the exception of not splitting user-visible strings, for grepping

Patch reviewers now recommend not splitting long user-visible strings,
such as printk messages, even if they exceed 80 columns.  This avoids
breaking grep.  However, that recommendation did not actually appear
anywhere in Documentation/CodingStyle.

See, for example, the thread at
  http://news.gmane.org/find-root.php?message_id=%3c1312215262.11635.15.camel%40Joe%2dLaptop%3e

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>