Improve documentation of AV1LrPickStruct structure

This CL improves the documentation of AV1LrPickStruct structure as
suggested in https://aomedia-review.googlesource.com/c/aom/+/183521.

Change-Id: I3ab12df457fe78356240003497368887dec6c05f

av1/encoder/encoder.h

diff --git a/av1/encoder/encoder.h b/av1/encoder/encoder.h
index ee904a1..adec789 100644
--- a/av1/encoder/encoder.h
+++ b/av1/encoder/encoder.h
@@ -1658,7 +1658,9 @@
 } RestUnitSearchInfo;
 
 /*!
- * \brief Buffers used during loop restoration search.
+ * \brief Structure to hold search parameter per restoration unit and
+ * intermediate buffer of Wiener filter used in pick filter stage of Loop
+ * restoration.
  */
 typedef struct {
   /*!