Replace underflow with overflow in doc comments

The use of underflow is misleading.  Adding one to MAX and
subtracting one from MIN are both considered an overflow.

bitcoin/src/blockdata/script/instruction.rs

@@ -231,7 +231,7 @@ impl<'a> InstructionIndices<'a> {
         let prev_remaining = self.remaining_bytes();
         let prev_pos = self.pos;
         let instruction = next_fn(self)?;
-        // No underflow: there must be less remaining bytes now than previously
+        // No overflow: there must be less remaining bytes now than previously
         let consumed = prev_remaining - self.remaining_bytes();
         // No overflow: sum will never exceed slice length which itself can't exceed `usize`
         self.pos += consumed;

bitcoin/src/taproot/serialized_signature.rs

@@ -217,7 +217,7 @@ mod into_iter {
 
         #[inline]
         fn size_hint(&self) -> (usize, Option<usize>) {
-            // can't underflow thanks to the invariant
+            // can't overflow thanks to the invariant
             let len = self.signature.len() - self.pos;
             (len, Some(len))
         }

units/src/amount/mod.rs

@@ -588,8 +588,8 @@ enum DisplayStyle {
 
 /// Calculates the sum over the iterator using checked arithmetic.
 pub trait CheckedSum<R>: sealed::Sealed<R> {
-    /// Calculates the sum over the iterator using checked arithmetic. If an over or underflow would
-    /// happen it returns [`None`].
+    /// Calculates the sum over the iterator using checked arithmetic. If an
+    /// overflow happens it returns [`None`].
     fn checked_sum(self) -> Option<R>;
 }
 

units/src/amount/signed.rs

@@ -25,10 +25,10 @@ use super::{
 /// Warning!
 ///
 /// This type implements several arithmetic operations from [`core::ops`].
-/// To prevent errors due to overflow or underflow when using these operations,
+/// To prevent errors due to an overflow when using these operations,
 /// it is advised to instead use the checked arithmetic methods whose names
 /// start with `checked_`. The operations from [`core::ops`] that [`SignedAmount`]
-/// implements will panic when overflow or underflow occurs.
+/// implements will panic when an overflow occurs.
 ///
 /// # Examples
 ///