gohcl: document optional attribute

This documents the "optional" attribute, which allows you to specify an
attribute that is optional when parsing.

gohcl/doc.go

@@ -17,6 +17,7 @@
 // attr (the default) indicates that the value is to be populated from an attribute
 // block indicates that the value is to populated from a block
 // label indicates that the value is to populated from a block label
+// optional is the same as attr, but the field is optional
 // remain indicates that the value is to be populated from the remaining body after populating other fields
 //
 // "attr" fields may either be of type *hcl.Expression, in which case the raw
@@ -34,6 +35,9 @@
 // the blocks being decoded. In this case, the name token is used only as
 // an identifier for the label in diagnostic messages.
 //
+// "optional" fields behave like "attr" fields, but they are optional
+// and will not give parsing errors if they are missing.
+//
 // "remain" can be placed on a single field that may be either of type
 // hcl.Body or hcl.Attributes, in which case any remaining body content is
 // placed into this field for delayed processing. If no "remain" field is