Added documentation of backslash ending string literals.

Closes rust-lang#22698

Author: jxcl

reference.md

@@ -302,7 +302,7 @@ nonzero_dec: '1' | '2' | '3' | '4'
 
 A _character literal_ is a single Unicode character enclosed within two
 `U+0027` (single-quote) characters, with the exception of `U+0027` itself,
-which must be _escaped_ by a preceding U+005C character (`\`).
+which must be _escaped_ by a preceding `U+005C` character (`\`).
 
 ##### String literals
 
@@ -311,6 +311,19 @@ A _string literal_ is a sequence of any Unicode characters enclosed within two
 which must be _escaped_ by a preceding `U+005C` character (`\`), or a _raw
 string literal_.
 
+A multi-line string literal may be defined by terminating each line with a
+`U+005C` character (`\`) immediately before the newline. This causes the
+`U+005C` character, the newline, and all whitespace at the beginning of the
+next line to be ignored.
+
+```rust
+let a = "foobar";
+let b = "foo\
+         bar";
+```
+
+`a` is equivalent to `b`.
+
 ##### Character escapes
 
 Some additional _escapes_ are available in either character or non-raw string