docs: explain that semicolons separate function arguments (#3685)

The manual's "Defining Functions" section shows only one-argument
functions, so it never says how to pass more than one. This adds a short
paragraph: arguments are separated by `;` in both `def f(a; b):` and
`f(.x; .y)`, and `f(.x, .y)` instead calls a one-argument `f` with the
filter `.x, .y`. It also adds an example for each. `tests/man.test` and
`jq.1.prebuilt` are regenerated.

Fixes #2163

tests/man.test

@@ -888,6 +888,14 @@ def addvalue(f): f as $x | map(. + $x); addvalue(.[0])
 [[1,2],[10,20]]
 [[1,2,1,2], [10,20,1,2]]
 
+def f(a; b): [a, b]; f(.x; .y)
+{"x":1,"y":2}
+[1,2]
+
+def f(a): [a]; f(.x, .y)
+{"x":1,"y":2}
+[1,2]
+
 isempty(empty)
 null
 true