Document behavior of to_pretty_string when passing Value

[marcospb19]

From https://docs.rs/serde_json/1.0.68/serde_json/fn.to_string_pretty.html:

I assume that if I call

use serde_json::*;
to_string_pretty<Value>(...);

I can .unwrap() this result safely, is it right? Or Value's implementation of Serialize might decide to fail?

If it's the former, can I make a PR adding this piece of info to this doc page?

[kangalio]

Looking at the source code, it seems Value's Serialize impl indeed cannot fail:

json/src/value/ser.rs

Lines 13 to 36 in 931ee23

	impl Serialize for Value {
	#[inline]
	fn serialize<S>(&self, serializer: S) -> result::Result<S::Ok, S::Error>
	where
	S: ::serde::Serializer,
	{
	match self {
	Value::Null => serializer.serialize_unit(),
	Value::Bool(b) => serializer.serialize_bool(*b),
	Value::Number(n) => n.serialize(serializer),
	Value::String(s) => serializer.serialize_str(s),
	Value::Array(v) => v.serialize(serializer),
	#[cfg(any(feature = "std", feature = "alloc"))]
	Value::Object(m) => {
	use serde::ser::SerializeMap;
	let mut map = tri!(serializer.serialize_map(Some(m.len())));
	for (k, v) in m {
	tri!(map.serialize_entry(k, v));
	}
	map.end()
	}
	}
	}
	}

I think a PR to state this in the docs is a good idea, although the docs should be added to Value, not to_string_pretty.