Fix some docstrings.

* helm-dabbrev.el (helm-dabbrev--same-major-mode-p): Do it.
* helm-imenu.el (helm-imenu-all-buffer-assoc):       Do it.
* helm-lib.el (helm-same-major-mode-p):              Do it.

helm-dabbrev.el

@@ -120,6 +120,7 @@ but the initial search for all candidates in buffer(s)."
         collect buf))
 
 (defun helm-dabbrev--same-major-mode-p (start-buffer)
+  "Decide if current-buffer is related to START-BUFFER."
   (helm-same-major-mode-p start-buffer helm-dabbrev-major-mode-assoc))
 
 (defun helm-dabbrev--collect (str limit ignore-case all)

helm-imenu.el

@@ -47,7 +47,7 @@
 
 (defcustom helm-imenu-all-buffer-assoc '((mu4e-view-mode . emacs-lisp-mode)
                                          (mu4e-compose-mode . emacs-lisp-mode))
-  "Major mode association alist.
+  "Major mode association alist for `helm-imenu-in-all-buffers'.
 Allow `helm-imenu-in-all-buffers' searching in these associated buffers
 even if they are not derived from each other."
   :type '(alist :key-type symbol :value-type symbol)

helm-lib.el

@@ -493,6 +493,8 @@ See `kill-new' for argument REPLACE."
 ;;; Modes
 ;;
 (defun helm-same-major-mode-p (start-buffer alist)
+  "Decide if current-buffer is related to START-BUFFER.
+Argument ALIST is an alist of associated major modes."
   ;; START-BUFFER is the current-buffer where we start searching.
   ;; Determine the major-mode of START-BUFFER as `cur-maj-mode'.
   ;; Each time the loop go in another buffer we try to find if its