headers: Some trivial fixes for some documentation typos

Note also the willneed/will_need inconsistency. I guess it could be nice to ASAP
choose one of them and introduce a backward compatibility hack for the other.

The issues was mostly found with:
for a in $(grep -r '^[ /]\*.*()' $(
	find -name '*.[ch]') |
	sed 's,^.* \([^ ]*\)().*$,\1,g' |
	sort |
	uniq |
	grep ^pa_)
do
	grep -rq "^.[^*].*\<$a(" $(find * -name '*.h') || echo $a
done

src/pulsecore/aupdate.h

@@ -39,7 +39,7 @@ void pa_aupdate_write_end(pa_aupdate *a);
 
 /* Will return 0, or 1, depending which copy of the data the caller
  * should modify. Each time called this will return the opposite of
- * the previous pa_aupdate_write_begin()/pa_aupdate_write_swap()
+ * the previous pa_aupdate_write_begin() / pa_aupdate_write_swap()
  * call. Should only be called between pa_aupdate_write_begin() and
  * pa_aupdate_write_end() */
 unsigned pa_aupdate_write_swap(pa_aupdate *a);