add brief doc to some macro (#2523)

* add doc

* Update lib.rs

substrate/srml/support/src/lib.rs

@@ -66,6 +66,9 @@ pub use runtime_io::print;
 #[doc(inline)]
 pub use srml_support_procedural::decl_storage;
 
+/// Return Err of the expression: `return Err($expression);`.
+///
+/// Used as `fail!(expression)`.
 #[macro_export]
 macro_rules! fail {
 	( $y:expr ) => {{
@@ -73,6 +76,9 @@ macro_rules! fail {
 	}}
 }
 
+/// Evaluate `$x:expr` and if not true return `Err($y:expr)`.
+///
+/// Used as `ensure!(expression_to_ensure, expression_to_return_on_false)`.
 #[macro_export]
 macro_rules! ensure {
 	( $x:expr, $y:expr ) => {{
@@ -82,6 +88,10 @@ macro_rules! ensure {
 	}}
 }
 
+/// Evaluate an expression, assert it returns an expected `Err` value and that
+/// runtime storage has not been mutated (i.e. expression is a no-operation).
+///
+/// Used as `assert_noop(expression_to_assert, expected_error_expression)`.
 #[macro_export]
 #[cfg(feature = "std")]
 macro_rules! assert_noop {
@@ -92,6 +102,13 @@ macro_rules! assert_noop {
 	}
 }
 
+/// Panic if an expression doesn't evaluate to an `Err`.
+///
+/// Used as `assert_err!(expression_to_assert, expected_err_expression)`.
+
+/// Assert an expression returns an error specified.
+///
+/// Used as `assert_err!(expression_to_assert, expected_error_expression)`
 #[macro_export]
 #[cfg(feature = "std")]
 macro_rules! assert_err {
@@ -100,6 +117,10 @@ macro_rules! assert_err {
 	}
 }
 
+/// Panic if an expression doesn't evaluate to `Ok`.
+///
+/// Used as `assert_ok!(expression_to_assert, expected_ok_expression)`,
+/// or `assert_ok!(expression_to_assert)` which would assert against `Ok(())`.
 #[macro_export]
 #[cfg(feature = "std")]
 macro_rules! assert_ok {