Document the relationship between octal numbers and the leading zero

Fixes ansible#37875

lib/ansible/utils/module_docs_fragments/files.py

@@ -23,9 +23,12 @@ class ModuleDocFragment(object):
 options:
   mode:
     description:
-      - Mode the file or directory should be. For those used to I(/usr/bin/chmod) remember that modes are actually octal numbers (like C(0644) or C(01777)).
-        Leaving off the leading zero will likely have unexpected results.
-        As of version 1.8, the mode may be specified as a symbolic mode (for example, C(u+rwx) or C(u=rw,g=r,o=r)).
+      - "Mode the file or directory should be. For those used to I(/usr/bin/chmod) remember that modes are actually octal numbers.
+        You must either specify the leading zero so that Ansible's YAML parser knows it is an octal
+        number (like C(0644) or C(01777)) or quote it (like C('644') or C('0644') so Ansible
+        receives a string and can do its own conversion from string into number.  Giving Ansible a number
+        without following one of these rules will end up with a decimal number which will have unexpected results.
+        As of version 1.8, the mode may be specified as a symbolic mode (for example, C(u+rwx) or C(u=rw,g=r,o=r))."
   owner:
     description:
       - Name of the user that should own the file/directory, as would be fed to I(chown).