Update spendable balance and ED spec

[gui1117]

I think the doc is wrong in regards to the spendable amount: the formula is wrong in some situations.
Also it says that reserved balances add a provider but the reality in the code is different:

#[test]
fn foo() {
	ExtBuilder::default().existential_deposit(10).build_and_execute_with(|| {
		let acc = 3456;
		Balances::set_balance(&acc, 100);
		Balances::set_balance(&2, 100);
		assert!(Balances::hold(&TestId::Foo, &acc, 10).is_ok());
		assert!(Balances::hold(&TestId::Foo, &acc, 90).is_err()); // No provider reference.
		assert!(Balances::hold(&TestId::Foo, &acc, 80).is_ok());
	});
}
#[test]
fn foo2() {
	ExtBuilder::default().existential_deposit(10).build_and_execute_with(|| {
		let acc = 3456;
		Balances::set_balance(&acc, 100);
		Balances::set_balance(&2, 100);
		System::inc_providers(&acc);
		assert!(Balances::hold(&TestId::Foo, &acc, 10).is_ok());
		assert!(Balances::hold(&TestId::Foo, &acc, 90).is_ok()); // Some provider reference.
	});
}

I have to look deeper to double check, but this is probably more correct than before.

[midegdugarova]

thanks @gui1117 ! good catch

[gui1117]

don't merge as-is it was more a comment that a direct PR, I tried to improve the PR a bit, I am still not sure what is the exact spec, so I don't know what to write down.

Sometimes the account is dusted sometimes it is not. I couldn't find any consistency.