Merge #208: Correct internal decode docs

f7dd6e5 Correct internal `decode` docs (Elias Rohrer)

Pull request description:

  Previously, the field docs stated that the `Hrp` would be always lowercase. However, we only assert that it's not mixed-case, i.e., all upper-case characters are allowed.

ACKs for top commit:
  tcharding:
    ACK f7dd6e5
  apoelstra:
    ACK f7dd6e5; successfully ran local tests
  clarkmoody:
    ACK f7dd6e5

Tree-SHA512: 420a9b27e3e6f17067e6b399e91237a040bc79837c57215324bf1cbe83de9c175d585ea9ce72a95f33628fb71953020af42a9e02cfae1d18876ac40802155e1b

Author: apoelstra

src/primitives/decode.rs

@@ -114,7 +114,7 @@ const SEP: char = '1';
 /// ```
 #[derive(Debug)]
 pub struct UncheckedHrpstring<'s> {
-    /// The human-readable part, guaranteed to be lowercase ASCII characters.
+    /// The human-readable part, guaranteed to be ASCII characters and not mixed-case.
     hrp: Hrp,
     /// This is ASCII byte values of the parsed string, guaranteed to be valid bech32 characters.
     ///
@@ -332,7 +332,7 @@ impl<'s> UncheckedHrpstring<'s> {
 /// ```
 #[derive(Debug)]
 pub struct CheckedHrpstring<'s> {
-    /// The human-readable part, guaranteed to be lowercase ASCII characters.
+    /// The human-readable part, guaranteed to be ASCII characters and not mixed-case.
     hrp: Hrp,
     /// This is ASCII byte values of the parsed string, guaranteed to be valid bech32 characters.
     ///