Clarify doc generation error (#365)

If the generated docs don't match the existing docs,
CI fails with a confusing error message.
This commit clarifies the error so that contributor can resolve the problem.

Signed-off-by: David Enyeart <enyeart@us.ibm.com>

scripts/regenDocs

@@ -215,6 +215,6 @@ echo "Doc generation completed"
 # Only checking status under docs/source folder
 cd $docsdir
 if [[ $(git status . --porcelain --untracked-file=no) ]]; then
-    echo "ERROR: New readme files generated, commit changes before doing push"
+    echo "ERROR: New rst documentation files generated that don't match the existing docs, run `make docs` to re-generate the rst documentation files before pushing"
     exit 1
 fi