docs: fix arithmetic update-assignment equivalence (#3682)

The manual said `a op= b` is equivalent to `a |= . op b`. It is actually
equivalent to `(b as $b | a |= . op $b)`: the right-hand side gets `.`
as input, not the value being updated, and each value it outputs gives a
separate result.

This updates the description and adds two examples, `.a += .b` and `.a
+= (1,2)`. `tests/man.test` and `jq.1.prebuilt` are regenerated.

Fixes #2119, fixes #2407

tests/man.test

@@ -994,6 +994,15 @@ true
 {"foo": 42}
 {"foo": 43}
 
+.a += .b
+{"a": 1, "b": 2}
+{"a": 3, "b": 2}
+
+.a += (1,2)
+{"a": 0}
+{"a": 1}
+{"a": 2}
+
 .a = .b
 {"a": {"b": 10}, "b": 20}
 {"a":20,"b":20}