doc: tweak built-in functions section & move dump section here

[ntrel]

dump is mentioned in the interface docs and isn't really an 'advanced' feature. It belongs as a built-in function even though it has special semantics (just like it methods are still methods).

Add println subheading.
Make 'Printing custom types' a subheading of println.
Change print functions to take IStr, not string and note IStr is not public. Note that print functions can take other printable types.
Mention dump in built-in functions list.

[Larpon]

Although more correct, I think println(s IStr) might confuse newcomers 🤔 "What's IStr?" " Where's IStr defined?" <- that sort of questions will flow in I think 🤷‍♂️

[spytheman]

Change print functions to take IStr, not string and note IStr is not public.

They do take string however, not IStr - the conversion is compiler magic currently, the functions themselves are ordinary V functions, accepting ordinary string type values.

[spytheman]

vlib/builtin/builtin.c.v:130:pub fn eprintln(s string) {
vlib/builtin/builtin.c.v:154:pub fn eprint(s string) {
vlib/builtin/builtin.c.v:196:pub fn print(s string) {
vlib/builtin/builtin.c.v:213:pub fn println(s string) {

[ntrel]

OK, updated.