hashes: Improve std/alloc features

Improve std/alloc features by doing:

- Fix incorrect feature enabling: "std" should enable "internals/std"
  not "internals/alloc".
- Put the "alloc" feature under the "std" feature, same as in other
  crates.
- Remove the comment, devs should understand what dependencies are for.
  We don't have comments everywhere else so we don't really need this one.

hashes/Cargo.toml

@@ -14,12 +14,9 @@ exclude = ["tests", "contrib"]
 
 [features]
 default = ["std"]
-std = ["internals/alloc"]
-schemars = ["actual-schemars", "dyn-clone"]
-# If you disable std, you can still use a Write trait via the core2 feature.
-# You can also use ToHex via the alloc feature, as it requires Vec/String.
-# And you can still just disable std by disabling default features, without enabling these two.
+std = ["internals/std"]
 alloc = ["internals/alloc"]
+schemars = ["actual-schemars", "dyn-clone"]
 serde-std = ["serde/std"]
 
 [dependencies]