Update vignettes/datatable-faq.Rmd

Co-authored-by: Benjamin Schwendinger <[email protected]>

Author: venom1204

vignettes/datatable-faq.Rmd

@@ -620,7 +620,7 @@ DT[ , b := rnorm(5)]  # 'replace' integer column with a numeric column
 
 ## Reading data.table from RDS or RData file
 
-`*.RDS` and `*.RData` are file types which can store in-memory R objects on disk efficiently. However, storing `data.table` into a binary file loses its column over-allocation and `truelength()`. This isn't a big deal -- your `data.table` will be copied in memory on the next _by reference_ operation and throw a warning.  
+`*.RDS` and `*.RData` are file types which can store in-memory R objects on disk efficiently. However, storing `data.table` into a binary file loses its column over-allocation (see also `?truelength`). This isn't a big deal -- your `data.table` will be copied in memory on the next _by reference_ operation and throw a warning.  
 Therefore, it is recommended to call `setDT()` on each `data.table` loaded with `readRDS()` or `load()` calls to restore its internal attributes. If you only need to pre-allocate space for new columns, `setalloccol()` can also be used.  
 
 For more details, see `?setDT` and `?truelength`.