Description
The pulseIn documentation claims:
Gives up and returns 0 if no pulse starts within a specified time out.
https://www.arduino.cc/reference/en/language/functions/advanced-io/pulsein/
While pulseInLong states:
Returns the length of the pulse in microseconds or 0 if no complete pulse was received within the timeout.
https://www.arduino.cc/reference/en/language/functions/advanced-io/pulseinlong/
Looking at the code and this pull request, it is clear that pulseInLong describes the correct behaviour for both functions (i.e. the timeout specified is for the pulse to start and end).
Additionally neither page documents that the functions will wait for any existing pulse to end then measure the next pulse (e.g. if measuring a HIGH pulse while the pin is already HIGH, it will wait for LOW, HIGH, LOW; rather than just LOW).