Improve documentation for assertions (#16988)

* Improve documentation for assertions

* Apply suggestions

Fix tests/assert/tassert_c.nim

* Use runnableExamples

* Move runnableExamples to module scope

tests/assert/tassert_c.nim

@@ -6,9 +6,9 @@ discard """
 const expected = """
 tassert_c.nim(35)        tassert_c
 tassert_c.nim(34)        foo
-assertions.nim(30)       failedAssertImpl
-assertions.nim(23)       raiseAssert
-fatal.nim(53)            sysFatal"""
+assertions.nim(*)       failedAssertImpl
+assertions.nim(*)       raiseAssert
+fatal.nim(*)            sysFatal"""
 
 proc tmatch(x, p: string): bool =
   var i = 0