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As a rule, API is written some time (usually two or three months) after release. Unfortunately, it also occurs that the old functionality is not described fully and correctly. For example, changes were made but not included in the documentation.
Thus, the only solution for the developer consists in reviewing the code itself. For these cases, there is a special free script Live API which scans current files of the kernel and displays a list of available API functions and events of all modules.