Add document for exported enums

There are several exported enums by libkmod without document, this patch
mainly added documentation for below enums like the way kmod_resources
be documented in.
* kmod_index
* kmod_remove
* kmod_insert
* kmod_probe
* kmod_filter
* kmod_module_initstate

This is not the best way to document these exported enums, however, it's
the simple way due to gtkdoc limits. It doesn't support export plain
enum like below: see https://bugzilla.gnome.org/show_bug.cgi?id=657444
---------8<-------head.h--------------8<-----------
...
enum foo {
    ...
};
...
---------8<-------end of head.h-------8<-----------
---------8<-------source.c------------8<-----------
...
/**
 * document for foo here
 */
...
typedef enum foo foo;
...
---------8<-------end of source.c-----8<----------
2 files changed