org-export: Make sure back-end specific filters do not make any back-end check

* contrib/lisp/org-e-ascii.el (org-e-ascii-filter-headline-blank-lines):
  Remove backend check.
* contrib/lisp/org-e-texinfo.el (org-e-texinfo-filter-section-blank-lines):
  Remove backend check.
* contrib/lisp/org-export.el (org-export-define-backend): Update
  docstring accordingly.

contrib/lisp/org-e-ascii.el

@@ -1726,7 +1726,7 @@ This function only applies to `e-ascii' back-end.  See
 `org-e-ascii-headline-spacing' for information.
 
 For any other back-end, HEADLINE is returned as-is."
-  (if (not (and (eq back-end 'e-ascii) org-e-ascii-headline-spacing)) headline
+  (if (not org-e-ascii-headline-spacing) headline
     (let ((blanks (make-string (1+ (cdr org-e-ascii-headline-spacing)) ?\n)))
       (replace-regexp-in-string "\n\\(?:\n[ \t]*\\)*\\'" blanks headline))))
 

contrib/lisp/org-e-texinfo.el

@@ -420,9 +420,8 @@ file name, %b by the file base name \(i.e without extension) and
 
 (defun org-e-texinfo-filter-section-blank-lines (headline back-end info)
   "Filter controlling number of blank lines after a section."
-  (if (not (eq back-end 'e-texinfo)) headline
-    (let ((blanks (make-string 2 ?\n)))
-      (replace-regexp-in-string "\n\\(?:\n[ \t]*\\)*\\'" blanks headline))))
+  (let ((blanks (make-string 2 ?\n)))
+    (replace-regexp-in-string "\n\\(?:\n[ \t]*\\)*\\'" blanks headline)))
 
 (defun org-e-texinfo--find-copying (info)
   "Retrieve the headline identified by the property :copying:.

contrib/lisp/org-export.el

@@ -745,7 +745,9 @@ keywords are understood:
 
     Alist between filters and function, or list of functions,
     specific to the back-end.  See `org-export-filters-alist' for
-    a list of all allowed filters.
+    a list of all allowed filters.  Filters defined here
+    shouldn't make a back-end test, as it may prevent back-ends
+    derived from this one to behave properly.
 
   :options-alist