docs: do not call integer overflows as underflows

src/libstd/io/buffered.rs

@@ -263,7 +263,7 @@ impl<R: Seek> Seek for BufReader<R> {
     /// See `std::io::Seek` for more details.
     ///
     /// Note: In the edge case where you're seeking with `SeekFrom::Current(n)`
-    /// where `n` minus the internal buffer length underflows an `i64`, two
+    /// where `n` minus the internal buffer length overflows an `i64`, two
     /// seeks will be performed instead of one. If the second seek returns
     /// `Err`, the underlying reader will be left at the same position it would
     /// have if you seeked to `SeekFrom::Current(0)`.

src/libstd/time/duration.rs

@@ -290,7 +290,7 @@ impl Duration {
     }
 
     /// Checked `Duration` subtraction. Computes `self - other`, returning [`None`]
-    /// if the result would be negative or if underflow occurred.
+    /// if the result would be negative or if overflow occurred.
     ///
     /// [`None`]: ../../std/option/enum.Option.html#variant.None
     ///