UndocumentedApi: support unnamed-enum

[guwirth]

samples with valid documentation:

enum /*unnamed-enum*/ {
};

enum /*unnamed-enum*/ : int {
};

/** doc */
typedef enum /*unnamed-enum*/ {
} type_name_3;

typedef enum /*unnamed-enum*/ {
} type_name_4; ///< doc

/** doc */
typedef enum /*unnamed-enum*/ : int {
} type_name_5;

typedef enum /*unnamed-enum*/ : int {
} type_name_6; ///< doc

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This change is