String.capitalize

You're seeing just the function capitalize, go back to String module for more information.
Link to this function

capitalize(string, mode \\ :default)

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Specs

capitalize(t(), :default | :ascii | :greek | :turkic) :: t()

Converts the first character in the given string to uppercase and the remainder to lowercase according to mode.

mode may be :default, :ascii, :greek or :turkic. The :default mode considers all non-conditional transformations outlined in the Unicode standard. :ascii capitalizes only the letters A to Z. :greek includes the context sensitive mappings found in Greek. :turkic properly handles the letter i with the dotless variant.

Examples

iex> String.capitalize("abcd")
"Abcd"

iex> String.capitalize("fin")
"Fin"

iex> String.capitalize("olá")
"Olá"