Improve 'dropped here' note.

Start mentioning function name that the variable is valid within in
notes to provide context.

src/test/ui/issues/issue-30438-c.nll.stderr

@@ -10,7 +10,7 @@ LL |     &x
    |     ^^ `x` would have to be valid for `'y`
 LL |     //~^ ERROR: `x` does not live long enough
 LL | }
-   | - but `x` dropped here while still borrowed
+   | - ...but `x` is only valid for the duration of the `silly` function, so it is dropped here while still borrowed
 
 error: aborting due to previous error