data-aggregator: document "onwrite" fault model requirements

The *-onwrite.sh analysis scripts only work if import-trace was not
run with --no-write-ecs, i.e. they only work if writing memory
accesses were imported into the "trace" table.

Change-Id: Icb2ea4e72d2200c886d4f9074f2da0f9bfd6ac85

tools/analysis/data-aggregator/function-occurrences-onwrite.sh

@@ -8,6 +8,9 @@ corrupted, and completely disregards storage duration (e.g. time between write
 and subsequent read).  The metric is calculated from the results of a normal
 "inject-on-read" campaign, only using the FI results from memory accesses that
 directly follow a "write" access.
+
+Only works if write equivalence classes are also imported into the database
+(i.e. import-trace is not run with --no-write-ecs).
 EOT
 }
 MUST_FILTER=1