doc for :transfomer

One important tip I found about using :transformer is that if you prepend any text, you need to make sure the returned string have the proper text-property `org-marker` at position 0, or `org-agenda-goto` (usually bound to TAB that let you jump to the headline in file) would not work.

Since this is a must I think it's probably good idea to include in the doc? Following is an example

```
:transformer (let* ((effort (get-text-property 0 'effort it))
(org-marker (get-text-property 0 'org-marker it))
)
(concat (propertize (concat "[effort: " effort "]")
'face 'italic
'org-marker org-marker ;; (1)
)
it)
)

```
If remove line (1), in resulting org-agenda buffer, TAB on those lines will show in message `org-agenda-error: Command not allowed in this line`; if with, will behave as intended and jump to the corresponding position.