Merge rust-bitcoin#3647: units: Comment alloc feature

17ca501 units: Comment alloc feature (Tobin C. Harding)

Pull request description:

  Add a comment to the `serde` module about why we have all the `alloc` feature gating.

  Done as part of rust-bitcoin#3556 - auditing the whole crate for use of `alloc` feature.

ACKs for top commit:
  apoelstra:
    ACK 17ca501; successfully ran local tests

Tree-SHA512: 6d13129903a9c530db811e9f64ae4fa653dbcc952a42502c53c6c622a80c1e58b3037edd0638de78867823ca370b3a4595a08a9ec07b84c71afb6200454f5d6d

units/src/amount/serde.rs

@@ -24,7 +24,7 @@ use core::fmt;
 
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
-#[cfg(feature = "alloc")]
+#[cfg(feature = "alloc")] // This is because `to_float_in` uses `to_string`.
 use super::Denomination;
 use super::{Amount, ParseAmountError, SignedAmount};