Document get_account() gotcha (#13713)

(cherry picked from commit f9acbd6e3f)

Co-authored-by: Michael Vines <mvines@gmail.com>

client/src/rpc_client.rs

@@ -610,6 +610,8 @@ impl RpcClient {
         }
     }
 
+    /// Note that `get_account` returns `Err(..)` if the account does not exist whereas
+    /// `get_account_with_commitment` returns `Ok(None)` if the account does not exist.
     pub fn get_account(&self, pubkey: &Pubkey) -> ClientResult<Account> {
         self.get_account_with_commitment(pubkey, self.commitment_config)?
             .value