added note about the disk space when creating backups

core/base_backup.go

@@ -31,6 +31,9 @@ const CacheKeyActiveBackup string = "@activeBackup"
 // The backup is executed within a transaction, meaning that new writes
 // will be temporary "blocked" until the backup file is generated.
 //
+// To safely perform the backup, it is recommended to have free disk space
+// for at least 2x the size of the pb_data directory.
+//
 // By default backups are stored in pb_data/backups
 // (the backups directory itself is excluded from the generated backup).
 //