Clarify "string continue" for (byte) string literals

The previous version just said "whitespace at the beginning of the next
line is ignored", but that is not quite correct. Currently, exactly four
characters are ignored in that position. This is different from the
definition of `char::is_whitespace` and `char::is_ascii_whitespace`.

Additionally "at the beginning of the next line" is confusing as
additional \n are also ignored.

https://github.com/rust-lang/rust/blob/595088d602049d821bf9a217f2d79aea40715208/compiler/rustc_lexer/src/unescape.rs#L281-L287

https://github.com/rust-lang/rust/blob/595088d602049d821bf9a217f2d79aea40715208/compiler/rustc_lexer/src/unescape.rs#L300-L307

src/tokens.md

@@ -153,16 +153,21 @@ which must be _escaped_ by a preceding `U+005C` character (`\`).
 Line-breaks are allowed in string literals. A line-break is either a newline
 (`U+000A`) or a pair of carriage return and newline (`U+000D`, `U+000A`). Both
 byte sequences are normally translated to `U+000A`, but as a special exception,
-when an unescaped `U+005C` character (`\`) occurs immediately before the
-line-break, then the `U+005C` character, the line-break, and all whitespace at the
-beginning of the next line are ignored. Thus `a` and `b` are equal:
+when an unescaped `U+005C` character (`\`) occurs immediately before a `\n`
+(`U+000A`), then the `U+005C` character, and all immediately following
+` ` (`U+0020`), `\t` (`U+0009`), `\n` (`U+000A`) and `\r` (`U+0000D`) characters
+are ignored. Thus `a`, `b` and `c` are equal:
 
 ```rust
 let a = "foobar";
 let b = "foo\
          bar";
+let c = "foo\
 
-assert_eq!(a,b);
+     bar";
+
+assert_eq!(a, b);
+assert_eq!(b, c);
 ```
 
 #### Character escapes