fix direct copy guidance (#18589)

* fix direct copy guidance

src/current/v23.2/molt-fetch.md

@@ -329,7 +329,12 @@ A local file server can be used with either the [`IMPORT INTO` or `COPY FROM` mo
 
 `--direct-copy` specifies that MOLT Fetch should use `COPY FROM` to move the source data directly to CockroachDB without an intermediate store:
 
-- Because the data is held in memory, the machine must have sufficient RAM for the amount of data being moved.
+- Because the data is held in memory, the machine must have sufficient RAM for the data currently in flight: 
+
+	~~~
+	average size of each row * --row-batch-size * --export-concurrency * --table-concurrency
+	~~~
+
 - Direct copy mode does not support compression or [continuation](#fetch-continuation).
 - The [`--use-copy`](#fetch-mode) flag is redundant with `--direct-copy`.
 

src/current/v24.1/molt-fetch.md

@@ -329,7 +329,12 @@ A local file server can be used with either the [`IMPORT INTO` or `COPY FROM` mo
 
 `--direct-copy` specifies that MOLT Fetch should use `COPY FROM` to move the source data directly to CockroachDB without an intermediate store:
 
-- Because the data is held in memory, the machine must have sufficient RAM for the amount of data being moved.
+- Because the data is held in memory, the machine must have sufficient RAM for the data currently in flight: 
+
+	~~~
+	average size of each row * --row-batch-size * --export-concurrency * --table-concurrency
+	~~~
+
 - Direct copy mode does not support compression or [continuation](#fetch-continuation).
 - The [`--use-copy`](#fetch-mode) flag is redundant with `--direct-copy`.