(byte-compile-dest-file, byte-compile-file): Doc fixes.

lisp/emacs-lisp/bytecomp.el

@@ -219,7 +219,9 @@ if you change this variable."
     ;; The user may want to redefine this along with emacs-lisp-file-regexp,
     ;; so only define it if it is undefined.
     (defun byte-compile-dest-file (filename)
-      "Convert an Emacs Lisp source file name to a compiled file name."
+      "Convert an Emacs Lisp source file name to a compiled file name.
+If FILENAME matches `emacs-lisp-file-regexp' (by default, files
+with the extension `.el'), add `c' to it; otherwise add `.elc'."
       (setq filename (byte-compiler-base-file-name filename))
       (setq filename (file-name-sans-versions filename))
       (cond ((eq system-type 'vax-vms)
@@ -1615,7 +1617,8 @@ This is normally set in local file variables at the end of the elisp file:
 ;;;###autoload
 (defun byte-compile-file (filename &optional load)
   "Compile a file of Lisp code named FILENAME into a file of byte code.
-The output file's name is made by appending `c' to the end of FILENAME.
+The output file's name is generated by passing FILENAME to the
+`byte-compile-dest-file' function (which see).
 With prefix arg (noninteractively: 2nd arg), LOAD the file after compiling.
 The value is non-nil if there were no errors, nil if errors."
 ;;  (interactive "fByte compile file: \nP")