lib/logstorage/consts.go: document that it isn't recommended setting maxColumnsPerBlock constant to too big values

This should help avoiding cases like this one - https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6425#issuecomment-2337446083

Author: valyala

lib/logstorage/consts.go

@@ -14,6 +14,10 @@ const maxUncompressedBlockSize = 2 * 1024 * 1024
 const maxRowsPerBlock = 8 * 1024 * 1024
 
 // maxColumnsPerBlock is the maximum number of columns per block.
+//
+// It isn't recommended setting this value to too big value, because this may result
+// in excess memory usage during data ingestion and significant slowdown during query execution,
+// since every column header is unpacked in every matching block during query execution.
 const maxColumnsPerBlock = 1_000
 
 // MaxFieldNameSize is the maximum size in bytes for field name.