parser: improve error message for fn (a Foo<Foo<T>>) baz(){}

vlib/v/parser/parse_type.v

@@ -682,7 +682,7 @@ pub fn (mut p Parser) parse_generic_inst_type(name string) ast.Type {
 			p.error_with_pos('cannot use multi return as generic concrete type', type_pos)
 		}
 		if !is_instance && gts.name.len > 1 {
-			p.error_with_pos('generic struct parameter name needs to be exactly one char',
+			p.error_with_pos('the parameter type name of a generic struct, must be a single capital letter placeholder name, like T or X, or a non-generic type name like int, string, etc.',
 				type_pos)
 		}
 		bs_name += gts.name

vlib/v/parser/tests/generic_struct_parameter_err.out

@@ -1,4 +1,4 @@
-vlib/v/parser/tests/generic_struct_parameter_err.vv:10:17: error: generic struct parameter name needs to be exactly one char
+vlib/v/parser/tests/generic_struct_parameter_err.vv:10:17: error: the parameter type name of a generic struct, must be a single capital letter placeholder name, like T or X, or a non-generic type name like int, string, etc.
     8 | struct MyContainer<T> {
     9 | mut:
    10 |     lst LinkedList<MyNode<T>>

vlib/v/parser/tests/generic_struct_receiver_nested_generic_err.out

@@ -0,0 +1,5 @@
+vlib/v/parser/tests/generic_struct_receiver_nested_generic_err.vv:5:11: error: the parameter type name of a generic struct, must be a single capital letter placeholder name, like T or X, or a non-generic type name like int, string, etc.
+    3 | }
+    4 | 
+    5 | fn (a Foo<Foo<T>>) baz() {}
+      |           ~~~~~~

vlib/v/parser/tests/generic_struct_receiver_nested_generic_err.vv

@@ -0,0 +1,5 @@
+struct Foo<T> {
+	bar T
+}
+
+fn (a Foo<Foo<T>>) baz() {}