💥 revise hook/var fns naming convention (2/2)

This is second of three big naming convention changes. In this commit,
we change the naming conventions for hook functions and variable
functions:

1. Replace the bar | to indicate a hook function with a -h suffix, e.g.

     doom|init-ui -> doom-init-ui-h
     doom|run-local-var-hooks -> doom-run-local-var-hooks-h

2. And add a -fn suffix for functions meant to be set on variables,
   e.g.

     (setq magit-display-buffer-function #'+magit-display-buffer-fn)

See ccf327f8 for the reasoning behind these changes.

core/autoload/buffers.el

@@ -250,7 +250,7 @@ regex PATTERN. Returns the number of killed buffers."
 ;; Hooks
 
 ;;;###autoload
-(defun doom|mark-buffer-as-real ()
+(defun doom-mark-buffer-as-real-h ()
   "Hook function that marks the current buffer as real."
   (doom-set-buffer-real (current-buffer) t))