Doc: Clarify newline escaping in triple-quoted strings (#160886)

Clarify that unescaped newlines are preserved in triple-quoted strings and
that `"\<newline>"` omits them in non-raw string literals.
Retain string literal concatenation as an alternative.

Author: prodehghan

Doc/reference/lexical_analysis.rst

@@ -838,8 +838,11 @@ A backslash can be added at the end of a line to ignore the newline::
    ... backslashes or newline characters.'
    'This string will not include backslashes or newline characters.'
 
-The same result can be achieved using :ref:`triple-quoted strings <strings>`,
-or parentheses and :ref:`string literal concatenation <string-concatenation>`.
+This escape sequence also works in non-raw :ref:`triple-quoted strings <strings>`,
+where unescaped newlines are otherwise preserved. It allows long string
+literals to be split across source lines without adding newline characters
+to the resulting string. Alternatively, the same result can be achieved
+using parentheses and :ref:`string literal concatenation <string-concatenation>`.
 
 .. _string-escape-escaped-char: