Document `decl_storage!` hashers (#3170)

substrate/srml/support/procedural/src/lib.rs

@@ -83,6 +83,14 @@ use proc_macro::TokenStream;
 ///   If the second key is untrusted, a cryptographic `hasher` such as `blake2_256` must be used.
 ///   Otherwise, other items in storage with the same first key can be compromised.
 ///
+/// Supported hashers (ordered from least to best security):
+///
+/// * `twox_64_concat` - TwoX with 64bit + key concatenated.
+/// * `twox_128` - TwoX with 128bit.
+/// * `twox_256` - TwoX with with 256bit.
+/// * `blake2_128` - Blake2 with 128bit.
+/// * `blake2_256` - Blake2 with 256bit.
+///
 /// Basic storage can be extended as such:
 ///
 /// `#vis #name get(#getter) config(#field_name) build(#closure): #type = #default;`