api docs: document that BUG() emits a trace2 error event

Correct documentation added in e544221d97 (trace2:
Documentation/technical/api-trace2.txt, 2019-02-22) to state that
calling BUG() also emits an "error" event. See ee4512ed48 (trace2:
create new combined trace facility, 2019-02-22) for the initial
implementation.

The BUG() function did not emit an event then however, that was only
changed later in 0a9dde4a04 (usage: trace2 BUG() invocations,
2021-02-05), that commit changed the code, but didn't update any of
the docs.

Let's also add a cross-reference from api-error-handling.txt.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-error-handling.txt

@@ -23,6 +23,9 @@ various kinds.
   without running into too many problems.  Like `error`, it
   returns -1 after reporting the situation to the caller.
 
+These reports will be logged via the trace2 facility. See the "error"
+event in link:api-trace2.txt[trace2 API].
+
 Customizable error handlers
 ---------------------------
 

Documentation/technical/api-trace2.txt

@@ -465,7 +465,7 @@ completed.)
 ------------
 
 `"error"`::
-	This event is emitted when one of the `error()`, `die()`,
+	This event is emitted when one of the `BUG()`, `error()`, `die()`,
 	`warning()`, or `usage()` functions are called.
 +
 ------------