Espruino's functions are tagged such that documentation can be built from them:

/// Comment before
void function();

/** Big comment before */
void function();

void function(); ///< Comment after 

To build. go to the root of the Espruino project, install Doxygen, then run:@

doxygen Doxyfile

Most functions you care about will be in doxygen/html/globals_0x6a.html