Improve sly-sticker's documentation

* contrib/sly-stickers.el (sly-stickers-clear-defun-stickers)
(sly-stickers-clear-buffer-stickers)
(sly-stickers-clear-region-stickers)
(sly-stickers-delete-sticker-at-point)
(sly-stickers-maybe-add-sticker): Add docstring.
(sly-stickers-maybe-add-sticker): Add a hint in the echo area.
(sly-stickers-fetch)
(sly-stickers-compile-region-aware-of-stickers): Add doctring.

* NEWS.md: describe `sly-stickers' a little more.