[Bug/Update] Update serialEvent() doc

I recently fell into the serialEvent() hole - the reference doc says: "Called when data is available. Use Serial.read() to capture this data."
The language here seems to suggest this is a callback - i.e. the function will be asynchronously called when data is available. Looking at the code, it is only called at the end of the loop on serialEventRun. This is dissimilar to a callback, because I was expecting it to be called even when in a while(true) loop in another function (as per what the doc suggests). The request here is to update the reference doc for serialEvent() so that others don't fall into the same hole and think it is actually a callback rather than a posthook function for loop().

Out of curiosity, what is the difference between serialEventRun and Serial.available() - is serialEventRun checkable within a sketch?

Locate the reference documentation for serialEvent() and compare its wording with the serialEventRun behavior at the end of loop(). Update the explanation so it does not imply asynchronous callback execution, and clarify the relevant relationship to Serial.available(); done when the reference accurately describes when serialEvent() runs.

Written by the indexing model from the issue text.