storage: Add comments for backend APIs

Just so it's clearer what to expect upon input and what types of return
values could be generated.  These were loosely copied from existing
virStorageBackendUpdateVolTargetInfoFD.

src/storage/storage_backend.c

@@ -1389,6 +1389,14 @@ static struct diskType const disk_types[] = {
 };
 
 
+/*
+ * virStorageBackendDetectBlockVolFormatFD
+ * @target: target definition ptr of volume to update
+ * @fd: fd of storage volume to update,
+ * @readflags: unused
+ *
+ * Returns 0 for success, -1 on a legitimate error condition
+ */
 static int
 virStorageBackendDetectBlockVolFormatFD(virStorageSourcePtr target,
                                         int fd,
@@ -1578,6 +1586,17 @@ virStorageBackendVolOpen(const char *path, struct stat *sb,
     return fd;
 }
 
+/*
+ * virStorageBackendUpdateVolTargetInfo
+ * @target: target definition ptr of volume to update
+ * @withBlockVolFormat: true if caller determined a block file
+ * @openflags: various VolOpenCheckMode flags to handle errors on open
+ * @readflags: unused
+ *
+ * Returns 0 for success, -1 on a legitimate error condition, and -2
+ * if the openflags used VIR_STORAGE_VOL_OPEN_NOERROR and some sort of
+ * open error occurred. It is up to the caller to handle.
+ */
 int
 virStorageBackendUpdateVolTargetInfo(virStorageSourcePtr target,
                                      bool withBlockVolFormat,
@@ -1636,6 +1655,17 @@ virStorageBackendUpdateVolTargetInfo(virStorageSourcePtr target,
     return ret;
 }
 
+/*
+ * virStorageBackendUpdateVolInfo
+ * @vol: Pointer to a volume storage definition
+ * @withBlockVolFormat: true if the caller determined a block file
+ * @openflags: various VolOpenCheckMode flags to handle errors on open
+ * @readflags: unused
+ *
+ * Returns 0 for success, -1 on a legitimate error condition, and -2
+ * if the openflags used VIR_STORAGE_VOL_OPEN_NOERROR and some sort of
+ * open error occurred. It is up to the caller to handle.
+ */
 int
 virStorageBackendUpdateVolInfo(virStorageVolDefPtr vol,
                                bool withBlockVolFormat,