doc: NEAR SDK Descriptions (#782)

Co-authored-by: NinoLipartiia4ire <[email protected]>
Co-authored-by: Austin Abell <[email protected]>

near-sdk/src/promise.rs

@@ -437,6 +437,23 @@ impl borsh::BorshSerialize for Promise {
     }
 }
 
+/// When the method can return either a promise or a value, it can be called with `PromiseOrValue::Promise`
+/// or `PromiseOrValue::Value` to specify which one should be returned.
+/// # Example
+/// ```no_run
+/// # use near_sdk::{ext_contract, near_bindgen, Gas, PromiseOrValue};
+/// #[ext_contract]
+/// pub trait ContractA {
+///     fn a(&mut self);
+/// }
+///
+/// let value = Some(true);
+/// let val: PromiseOrValue<bool> = if let Some(value) = value {
+///     PromiseOrValue::Value(value)
+/// } else {
+///     contract_a::a("bob_near".parse().unwrap(), 0, Gas(1_000)).into()
+/// };
+/// ```
 #[derive(serde::Serialize)]
 #[serde(untagged)]
 pub enum PromiseOrValue<T> {

near-sdk/src/test_utils/mod.rs

@@ -1,3 +1,5 @@
+//! Testing blockchain utilities. These can only be used inside tests and are not available for
+//! a wasm32 target.
 #[allow(dead_code)]
 pub mod test_env;
 

near-sdk/src/types/vm_types.rs

@@ -2,7 +2,9 @@
 pub use near_vm_logic::types::{PromiseResult as VmPromiseResult, ReturnData};
 
 //* Types from near_vm_logic
+/// Promise index that is computed only once.
 pub type PromiseIndex = u64;
+/// An index of Receipt to append an action
 pub type ReceiptIndex = u64;
 pub type IteratorIndex = u64;
 

near-sdk/src/utils/mod.rs

@@ -1,3 +1,5 @@
+//! Helper methods that often used in smart contracts.
+
 pub(crate) mod storage_key_impl;
 
 #[cfg(feature = "unstable")]