org.el (org-entry-blocked-p): Don't set the buffer as modified

org.el (org-entry-blocked-p): Don't set the buffer as modified

* org.el (org-blocker-hook): Update the docstring to mention
that functions in this hook should not modify the buffer.
(org-trigger-hook): Small docstring fix.
(org-entry-blocked-p): Use `with-buffer-modified-unmodified'
so that the function never modifies the buffer.

Thanks to Sven Bretfeld for reporting this.

When building the agenda, checking for blocked items should not
set the buffers as modified, otherwise exiting the agenda will ask
for confirmation each time it kills a buffer.