filter: add docs for `git_filter_stream_fn`

include/git2/sys/filter.h

@@ -167,14 +167,14 @@ typedef void GIT_CALLBACK(git_filter_shutdown_fn)(git_filter *self);
  *
  * The `payload` will be a pointer to a reference payload for the filter.
  * This will start as NULL, but `check` can assign to this pointer for
- * later use by the `apply` callback.  Note that the value should be heap
- * allocated (not stack), so that it doesn't go away before the `apply`
+ * later use by the `stream` callback.  Note that the value should be heap
+ * allocated (not stack), so that it doesn't go away before the `stream`
  * callback can use it.  If a filter allocates and assigns a value to the
  * `payload`, it will need a `cleanup` callback to free the payload.
  */
 typedef int GIT_CALLBACK(git_filter_check_fn)(
 	git_filter              *self,
-	void       **payload, /* points to NULL ptr on entry, may be set */
+	void                   **payload, /* NULL on entry, may be set */
 	const git_filter_source *src,
 	const char             **attr_values);
 
@@ -197,6 +197,15 @@ typedef int GIT_CALLBACK(git_filter_apply_fn)(
 	const git_buf           *from,
 	const git_filter_source *src);
 
+/**
+ * Callback to perform the data filtering.
+ *
+ * Specified as `filter.stream`, this is a callback that filters data
+ * in a streaming manner.  This function will provide a
+ * `git_writestream` that will the original data will be written to;
+ * with that data, the `git_writestream` will then perform the filter
+ * translation and stream the filtered data out to the `next` location.
+ */
 typedef int GIT_CALLBACK(git_filter_stream_fn)(
 	git_writestream        **out,
 	git_filter              *self,
@@ -208,9 +217,10 @@ typedef int GIT_CALLBACK(git_filter_stream_fn)(
  * Callback to clean up after filtering has been applied
  *
  * Specified as `filter.cleanup`, this is an optional callback invoked
- * after the filter has been applied.  If the `check` or `apply` callbacks
- * allocated a `payload` to keep per-source filter state, use this
- * callback to free that payload and release resources as required.
+ * after the filter has been applied.  If the `check`, `apply`, or
+ * `stream` callbacks allocated a `payload` to keep per-source filter
+ * state, use this callback to free that payload and release resources
+ * as required.
  */
 typedef void GIT_CALLBACK(git_filter_cleanup_fn)(
 	git_filter              *self,
@@ -248,21 +258,24 @@ struct git_filter {
 	/**
 	 * Called to determine whether the filter should be invoked for a
 	 * given file.  If this function returns `GIT_PASSTHROUGH` then the
-	 * `apply` function will not be invoked and the contents will be passed
-	 * through unmodified.
+	 * `stream` or `apply` functions will not be invoked and the
+	 * contents will be passed through unmodified.
 	 */
 	git_filter_check_fn    check;
 
 	/**
-	 * Called to actually apply the filter to file contents.  If this
-	 * function returns `GIT_PASSTHROUGH` then the contents will be passed
-	 * through unmodified.
+	 * Provided for backward compatibility; this will apply the
+	 * filter to the given contents in a `git_buf`.  Callers should
+	 * provide a `stream` function instead.
 	 */
 	git_filter_apply_fn    apply;
 
 	/**
-	 * Called to apply the filter in a streaming manner.  If this is not
-	 * specified then the system will call `apply` with the whole buffer.
+	 * Called to apply the filter, this function will provide a
+	 * `git_writestream` that will the original data will be
+	 * written to; with that data, the `git_writestream` will then
+	 * perform the filter translation and stream the filtered data
+	 * out to the `next` location.
 	 */
 	git_filter_stream_fn   stream;
 
@@ -289,9 +302,9 @@ GIT_EXTERN(int) git_filter_init(git_filter *filter, unsigned int version);
  * As mentioned elsewhere, the initialize callback will not be invoked
  * immediately.  It is deferred until the filter is used in some way.
  *
- * A filter's attribute checks and `check` and `apply` callbacks will be
- * issued in order of `priority` on smudge (to workdir), and in reverse
- * order of `priority` on clean (to odb).
+ * A filter's attribute checks and `check` and `stream` (or `apply`)
+ * callbacks will be issued in order of `priority` on smudge (to
+ * workdir), and in reverse order of `priority` on clean (to odb).
  *
  * Two filters are preregistered with libgit2:
  * - GIT_FILTER_CRLF with priority 0