Clarify tab can be escaped (or not) in basic strings

As discussed in #877, the current description of basic strings in the
docs say that tab characters are allowed. However this text is
immediately followed by an example that uses escaped tabs `\t`, which
make it a bit ambiguous and hard to understand.

The change introduced here explicitly says that tabs can be used either
directly or in its escaped form.

Author: abravalheri

toml.md

@@ -284,6 +284,8 @@ For convenience, some popular characters have a compact escape sequence.
 \UXXXXXXXX - unicode         (U+XXXXXXXX)
 ```
 
+Tab characters can be used either directly or in its escaped form `\t`.
+
 Any Unicode character may be escaped with the `\uXXXX` or `\UXXXXXXXX` forms.
 The escape codes must be valid Unicode [scalar
 values](https://unicode.org/glossary/#unicode_scalar_value).