doc: in description, replace field with header for csrf_* settings

The doc for the csrf_* settings referered to "field is missing" when
referencing it as headers elsewhere in the doc. Change to use header
instead of field to reduce confusion.

Author: rouilj

roundup/configuration.py

@@ -1348,7 +1348,7 @@ def str2value(self, value):
     the user if the header is missing or invalid.
 Set this to 'yes' to block the post and notify the user
     if the header is invalid, but accept the form if
-    the field is missing.
+    the header is missing.
 Set this to 'logfailure' to log a notice to the roundup
     log if the header is invalid or missing, but accept
     the post.
@@ -1360,7 +1360,7 @@ def str2value(self, value):
     the user if the header is missing or invalid.
 Set this to 'yes' to block the post and notify the user
     if the header is invalid, but accept the form if
-    the field is missing.
+    the header is missing.
 Set this to 'logfailure' to log a notice to the roundup
     log if the header is invalid or missing, but accept
     the post.
@@ -1391,7 +1391,7 @@ def str2value(self, value):
     the user if the header is missing or invalid.
 Set this to 'yes' to block the post and notify the user
     if the header is invalid, but accept the form if
-    the field is missing.
+    the header is missing.
 Set this to 'logfailure' to log a notice to the roundup
     log if the header is invalid or missing, but accept
     the post.
@@ -1404,7 +1404,7 @@ def str2value(self, value):
     the user if the header is missing or invalid.
 Set this to 'yes' to block the post and notify the user
     if the header is invalid, but accept the form if
-    the field is missing.
+    the header is missing.
 Set this to 'logfailure' to log a notice to the roundup
     log if the header is invalid or missing, but accept
     the post.