docs: add docs for unchecked_duration_subtraction lint

src/docs/unchecked_duration_subtraction.txt

@@ -0,0 +1,19 @@
+### What it does
+Finds patterns of unchecked subtraction of [`Duration`] from [`Instant::now()`].
+
+### Why is this bad?
+Unchecked subtraction could cause underflow on certain platforms, leading to bugs and/or
+unintentional panics.
+
+### Example
+```
+let time_passed = Instant::now() - Duration::from_secs(5);
+```
+
+Use instead:
+```
+let time_passed = Instant::now().checked_sub(Duration::from_secs(5));
+```
+
+[`Duration`]: std::time::Duration
+[`Instant::now()`]: std::time::Instant::now;