Clarify mode behavior for modules that can set file permissions (#72837)

lib/ansible/modules/copy.py

@@ -75,6 +75,11 @@ options:
     - As of Ansible 2.3, the mode may also be the special string C(preserve).
     - C(preserve) means that the file will be given the same permissions as the source file.
     - When doing a recursive copy, see also C(directory_mode).
+    - If C(mode) is not specified and the destination file B(does not) exist, the default C(umask) on the system will be used
+      when setting the mode for the newly created file.
+    - If C(mode) is not specified and the destination file B(does) exist, the mode of the existing file will be used.
+    - Specifying C(mode) is the best way to ensure files are created with the correct permissions.
+      See CVE-2020-1736 for further details.
   directory_mode:
     description:
     - When doing a recursive copy set the mode for the directories.

lib/ansible/plugins/doc_fragments/files.py

@@ -25,6 +25,11 @@ options:
       number which will have unexpected results.
     - As of Ansible 1.8, the mode may be specified as a symbolic mode (for example, C(u+rwx) or
       C(u=rw,g=r,o=r)).
+    - If C(mode) is not specified and the destination file B(does not) exist, the default C(umask) on the system will be used
+      when setting the mode for the newly created file.
+    - If C(mode) is not specified and the destination file B(does) exist, the mode of the existing file will be used.
+    - Specifying C(mode) is the best way to ensure files are created with the correct permissions.
+      See CVE-2020-1736 for further details.
     type: raw
   owner:
     description: