kernel-doc: single DOC: selection

Currently, DOC: sections are always output even if only a single
function is requested, fix this and also make it possible to just
output a single DOC: section by giving its title as the function
name to output.

Also fixes docbook XML well-formedness for sections with examples.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>