This is the 10245th most frequent Turkish word.
belgeselinin
"Belgeselinin" means "of your/his/her documentary."
In this sentence, 'belgeselinin' is used to indicate possession, specifically referring to the narration of the mentioned documentary.
Doğa belgeselinin seslendirmesi çok başarılıydı.
The narration of the nature documentary was very successful.
Here, 'belgeselinin' shows a possessive relationship, linking 'belgesel(inin)' to the phrase 'about his life.'
Onun hayatını anlatan belgeselinin çekimleri tamamlandı.
The filming of the documentary about his life is completed.
In this example, 'belgeselinin' denotes possession between the documentary and 'Ünlü bir yönetmen.'