Document the accuracy of the ledger timestamp (#1289)

### What
Document the accuracy of the ledger timestamp.

### Why
It's important that the accuracy is known by developers relying on the
timestamp for certain use cases. The update here is taken from a recent
update in the Stellar docs and communicates the same.

Related stellar/stellar-docs#509
Related stellar/stellar-docs#572

Close #1258

soroban-sdk/src/ledger.rs

@@ -3,6 +3,9 @@ use crate::{env::internal, unwrap::UnwrapInfallible, BytesN, Env, TryIntoVal};
 
 /// Ledger retrieves information about the current ledger.
 ///
+/// For more details about the ledger and the ledger header that the values in the Ledger are derived from, see:
+///  - https://developers.stellar.org/docs/learn/encyclopedia/network-configuration/ledger-headers
+///
 /// ### Examples
 ///
 /// ```
@@ -75,9 +78,12 @@ impl Ledger {
 
     /// Returns a unix timestamp for when the ledger was closed.
     ///
-    /// The timestamp is the number of seconds, excluding leap seconds,
-    /// that have elapsed since unix epoch. Unix epoch is January 1st, 1970,
-    /// at 00:00:00 UTC.
+    /// The timestamp is the number of seconds, excluding leap seconds, that
+    /// have elapsed since unix epoch. Unix epoch is January 1st, 1970, at
+    /// 00:00:00 UTC.
+    ///
+    /// For more details see:
+    ///  - https://developers.stellar.org/docs/learn/encyclopedia/network-configuration/ledger-headers#close-time
     pub fn timestamp(&self) -> u64 {
         internal::Env::get_ledger_timestamp(self.env())
             .unwrap_infallible()