Clarify date format docs

Fixes #3369

editions/tw5.com/tiddlers/features/DateFormat.tid

@@ -1,10 +1,10 @@
 created: 20140418142957325
-modified: 20170630223008226
+modified: 20180730201626985
 tags: Features
 title: DateFormat
 type: text/vnd.tiddlywiki
 
-The ViewWidget accepts a `template` attribute that allows the format of date values to be specified. The format string is processed with the following substitutions:
+When used to display date values (with the `format` attribute set to ''date''), the ViewWidget accepts a `template` attribute that allows the format of the date values to be specified. The format string is processed with the following substitutions:
 
 |!Token |!Substituted Value |
 |`DDD` |Day of week in full (eg, "Monday") |

editions/tw5.com/tiddlers/widgets/ViewWidget.tid

@@ -1,6 +1,6 @@
 caption: view
 created: 20131024141900000
-modified: 20170824195528248
+modified: 20180730201626985
 tags: Widgets
 title: ViewWidget
 type: text/vnd.tiddlywiki
@@ -18,7 +18,7 @@ The content of the `<$view>` widget is displayed if the field or property is mis
 |field |The name of the field to view (defaults to "text") |
 |index |The name of the index to view |
 |format |The format for displaying the field (see below) |
-|template |Optional template string used with certain formats such as dates |
+|template |Optional template string used when the `format` attribute is set to "date" |
 |subtiddler |Optional SubTiddler title when the target tiddler is a [[plugin|Plugins]] (see below) |
 |mode |<<.from-version "5.1.15">> Optional transclusion parsing mode for wikified formats. May be "inline" or "block" (the default) |