fix some more style-guide violations in the alert-messages

python/ql/src/Statements/DocStrings.ql

@@ -48,4 +48,4 @@ string scope_type(Scope s) {
 
 from Scope s
 where needs_docstring(s) and not exists(s.getDocString())
-select s, scope_type(s) + " " + s.getName() + " does not have a docstring"
+select s, scope_type(s) + " " + s.getName() + " does not have a docstring."

python/ql/src/Statements/NonIteratorInForLoop.ql

@@ -22,4 +22,4 @@ where
   not t.failedInference(_) and
   not v = Value::named("None") and
   not t.isDescriptorType()
-select loop, "$@ of class '$@' may be used in for-loop.", origin, "Non-iterable", t, t.getName()
+select loop, "$@ of class $@ may be used in for-loop.", origin, "Non-iterable", t, t.getName()

python/ql/src/Statements/SideEffectInAssert.ql

@@ -42,4 +42,4 @@ predicate probable_side_effect(Expr e) {
 
 from Assert a, Expr e
 where probable_side_effect(e) and a.contains(e)
-select a, "This 'assert' statement contains $@ which may have side effects.", e, "an expression"
+select a, "This 'assert' statement contains an $@ which may have side effects.", e, "expression"

python/ql/src/Statements/UnusedExceptionObject.ql

@@ -17,4 +17,4 @@ where
   call.getFunc().pointsTo(ex) and
   ex.getASuperType() = ClassValue::exception() and
   exists(ExprStmt s | s.getValue() = call)
-select call, "Instantiating an exception, but not raising it, has no effect"
+select call, "Instantiating an exception, but not raising it, has no effect."