fix #17615(runnableExamples silently ignored if placed after some code) (#17619)

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

tests/nimdoc/t17615.nim

@@ -0,0 +1,11 @@
+discard """
+  cmd: "nim doc -r $file"
+  errormsg: "runnableExamples must appear before the first non-comment statement"
+  line: 10
+"""
+
+func fn*() =
+  ## foo
+  discard
+  runnableExamples:
+    assert true