[cpp.stringize] Clarify distinct operations by introducing a comma

Co-authored-by: Jan Schultke <[email protected]>

Author: AlisdairM

source/preprocessor.tex

@@ -1798,7 +1798,8 @@
 contained within the preprocessing token:
 a \unicode{005c}{reverse solidus} character (\tcode{\textbackslash})
 is inserted before each \unicode{0022}{quotation mark} and
-\unicode{005c}{reverse solidus} character and each new-line character is
+\unicode{005c}{reverse solidus} character,
+and each new-line character is
 replaced by the two-character sequence \tcode{\textbackslash n}.
 If the replacement that results is not a valid character string literal,
 the program is ill-formed. The character string literal corresponding to