scripts: kernel_doc: fixup reporting of function identifiers

When function description includes brackets after the function name as
suggested by Documentation/doc-guide/kernel-doc, the kernel-doc script
omits the function name from "Scanning doc for" report.
Extending match for identifier name with optional brackets fixes this
issue.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>