A list of functions to be called at the start of `write-region'.
Each is passed two arguments, START and END as for `write-region'.
These are usually two numbers but not always; see the documentation
for `write-region'. The function should return a list of pairs
of the form (POSITION . STRING), consisting of strings to be effectively
inserted at the specified positions of the file being written (1 means to
insert before the first byte written). The POSITIONs must be sorted into
increasing order.
If there are several annotation functions, the lists returned by these
functions are merged destructively. As each annotation function runs,
the variable `write-region-annotations-so-far' contains a list of all
annotations returned by previous annotation functions.
An annotation function can return with a different buffer current.
Doing so removes the annotations returned by previous functions, and
resets START and END to `point-min' and `point-max' of the new buffer.
After `write-region' completes, Emacs calls the function stored in
`write-region-post-annotation-function', once for each buffer that was
current when building the annotations (i.e., at least once), with that
buffer current.