kernel/module: improve documentation for try_module_get()

The sentence "this call will fail if the module is already being
removed" is potentially confusing and may contradict the rest of the
documentation. If one tries to get a module that has already been
removed using a stale pointer, the kernel will crash.

Signed-off-by: Marco Pagani <marpagan@redhat.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>