Fix description of -W message field in man page (GH-111371)

(cherry picked from commit 9511198)

Co-authored-by: Ville Skyttä <ville.skytta@iki.fi>

Author: scop

Misc/python.man

@@ -285,7 +285,7 @@ the remaining fields.
 
 The
 .I message
-field must match the whole printed warning message; this match is
+field must match the start of the warning message; this match is
 case-insensitive.
 
 The