more restriction

Author: Jonathan Zong

site/docs/encoding.md

@@ -279,6 +279,7 @@ Note: `time` encoding animations currently have a few restrictions. See the [exa
   - parameter must select the same field as the `time` field definition
 - must explicitliy define a filter using that parameter
 - currently, the `time` channel only supports `band` scales (these are the default for discrete frame animation)
+- currently, only unit specifications are supported (no multi-view animations)
 
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 TODO(jzong): uncomment once we've added rescale, scale types