Add clarification of the r string modifier

Raw sting literals may not be easily understood based on the brief description. This description is much more verbose (maybe too verbose) but should help the novice python programmer understand what the r modifier is and how / when to use it.

doc/data/messages/a/anomalous-backslash-in-string/details.rst

@@ -1,2 +1,25 @@
 ``\z`` is same as ``\\z`` because there's no escape sequence for ``z``. But it is not clear
 for the reader of the code.
+
+Background
+
+Python's string literals use the backslash for their own escape
+sequences (like \n for a newline). When Python sees an escape sequence
+it doesn't recognize, such as "\." (a literal "dot" character in regex),
+it gives a DeprecationWarning.
+
+To avoid this warning, you can use a raw string literal for your regular
+expression. Raw string literals don't treat the backslash as a special
+character and are often used for regular expressions in Python.
+
+By adding the r before the string, you're telling Python to treat this
+as a raw string literal, so it won't try to interpret \. as an escape
+sequence and will instead ignore it.
+
+An alternative would be to use double backslash \\. The first backslash
+escapes the second one when the string is parsed by python. The second
+backslash works as the escape sequence for the . when sent to the regex
+engine for parsing. In other words, '\\.' becomes '\.' when parsed by
+python, and it can then be used in a regex. The "r" before the string
+tells python to treat the string as a "raw" string literal (and not to
+modify it).