kernel-doc: rename the kernel-doc directive 'functions' to 'identifiers'

The 'functions' directive is not only for functions, but also works for
structs/unions. So the name is misleading. This patch renames it to
'identifiers', which specific the functions/types to be included in
documentation. We keep the old name as an alias of the new one before
all documentation are updated.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>