Explain double quote backslash; closes #2597

src/ch12-04-testing-the-librarys-functionality.md

@@ -48,8 +48,10 @@ yet.
 function we wish we had</span>
 
 This test searches for the string `"duct"`. The text we’re searching is three
-lines, only one of which contains `"duct"`. We assert that the value returned
-from the `search` function contains only the line we expect.
+lines, only one of which contains `"duct"` (Note that the backslash after the
+opening double quote tells Rust not to put a newline character at the beginning
+of the contents of this string literal). We assert that the value returned from
+the `search` function contains only the line we expect.
 
 We aren’t able to run this test and watch it fail because the test doesn’t even
 compile: the `search` function doesn’t exist yet! So now we’ll add just enough