kernel-doc: describe the ``literal`` syntax

changeset b97f193a ("scripts/kernel-doc: fix parser
for apostrophes") added support for ``literal`` inside
kernel-doc, in order to allow using the "%" symbol inside
a literal block, as this is used at printk() description.

Document it.

Fixes: b97f193a ("scripts/kernel-doc: fix parser for apostrophes")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>