bpo-32614: Modify re examples to use a raw string to prevent warning (GH-5265)

Modify RE examples in documentation to use raw strings to prevent DeprecationWarning.
Add text to REGEX HOWTO to highlight the deprecation.  Approved by Serhiy Storchaka.
diff --git a/Doc/howto/unicode.rst b/Doc/howto/unicode.rst
index d4b8f8d..093f445 100644
--- a/Doc/howto/unicode.rst
+++ b/Doc/howto/unicode.rst
@@ -463,7 +463,7 @@
 Arabic numerals::
 
    import re
-   p = re.compile('\d+')
+   p = re.compile(r'\d+')
 
    s = "Over \u0e55\u0e57 57 flavours"
    m = p.search(s)