CODING_STYLE: document why O_NONBLOCK makes sense when opening regular files, too

doc/CODING_STYLE

@@ -440,3 +440,13 @@
   string, always apply the C-style unescaping fist, followed by the specifier
   expansion. When doing the reverse, make sure to escape '%' in specifier-style
   first (i.e. '%' → '%%'), and then do C-style escaping where necessary.
+
+- It's a good idea to use O_NONBLOCK when opening 'foreign' regular files, i.e
+  file system objects that are supposed to be regular files whose paths where
+  specified by the user and hence might actually refer to other types of file
+  system objects. This is a good idea so that we don't end up blocking on
+  'strange' file nodes, for example if the user pointed us to a FIFO or device
+  node which may block when opening. Moreover even for actual regular files
+  O_NONBLOCK has a benefit: it bypasses any mandatory lock that might be in
+  effect on the regular file. If in doubt consider turning off O_NONBLOCK again
+  after opening.