docs: Add underscore issue to FAQ

docs/faq.md

@@ -254,6 +254,13 @@ prevent masking an error for a situation when you expected a file to already be
 at that location. It also turns out to simplify the internals significantly, and
 remove an ambiguous scenario with the reversable file resource.
 
+### Why do function names inside of templates include underscores?
+
+The golang template library which we use to implement the template() function
+doesn't support the dot notation, so we import all our normal functions, and
+just replace dots with underscores. As an example, the standard `datetime.print`
+function is shown within mcl scripts as datetime_print after being imported.
+
 ### On startup `mgmt` hangs after: `etcd: server: starting...`.
 
 If you get an error message similar to:

examples/lang/datetime1.mcl

@@ -5,3 +5,7 @@ file "/tmp/mgmt/datetime" {
 	state => $const.res.file.state.exists,
 	content => template("Hello! It is now: {{ datetime_print . }}\n", $d),
 }
+
+file "/tmp/mgmt/" {
+	state => $const.res.file.state.exists,
+}