Clarify the usage of custom type

[c4710n]

Before, the doc uses mod.fun(values, ...args) to describe the usage of custom type. However, it failed to clearly explain the underlying approach, because ... is not a valid syntax of Elixir.

This PR try to clarify the usage by referring the underlying call.

Although covering the underlying implementation in the documentation is not ideal, it is likely the easiest way to understand how things work, especially since developers are indeed required to implement certain aspects here.

[coveralls]

Pull Request Test Coverage Report for Build 5c3e5232b809373b9d4bf7cc3960d2acbff4e0b5-PR-137

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.448%

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Totals
Change from base Build 1d269dc7b03b027be4d880d52b8ad8d406e4580e:	0.0%
Covered Lines:	271
Relevant Lines:	290

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💛 - Coveralls

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