ICU-20841 Update buildtool docs according to user feedback.
See #835
diff --git a/docs/userguide/icu_data/buildtool.md b/docs/userguide/icu_data/buildtool.md
index da4c859..c7d5451 100644
--- a/docs/userguide/icu_data/buildtool.md
+++ b/docs/userguide/icu_data/buildtool.md
@@ -45,10 +45,12 @@
ICU_DATA_FILTER_FILE=filters.json path/to/icu4c/source/runConfigureICU Linux
-You must have the data sources in order to use the ICU Data Build Tool.
-Check for the file icu4c/source/data/locales/root.txt. If that file is
-missing, you need to download "icu4c-*-data.zip" and replace the contents of
-icu4c/source/data with the data directory from the zip file.
+**Important:** You *must* have the data sources in order to use the ICU Data
+Build Tool. Check for the file icu4c/source/data/locales/root.txt. If that file
+is missing, you need to download "icu4c-\*-data.zip", delete the old
+icu4c/source/data directory, and replace it with the data directory from the zip
+file. If there is a \*.dat file in icu4c/source/data/in, that file will be used
+even if you gave ICU custom filter rules.
In order to use Hjson syntax, the `hjson` pip module must be installed on
your system. You should also consider installing the `jsonschema` module to
