docs(api-and-naming): note deprecated files having private API

Author: scop

doc/api-and-naming.md

@@ -19,6 +19,11 @@ each call that references the value, variable name, or their existence,
 typically by `unset -v`ing them when multiple such calls are used,
 to avoid them interfering with each other.
 
+Everything in deprecated completion files (ones starting with an underscore)
+is considered private and is to be named accordingly. Deprecated files are not
+intended to be explicitly used with `_comp_xfunc`, and completion files having
+precedence over them may have a different API.
+
 ## Naming
 
 Due to its nature, bash-completion adds a number of functions and variables in