add documentation

docs/WebUI/language-features.md

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+# PlantUML Language Features
+
+## Auto Completion
+
+### Icons
+
+- type `<&` to get a list of PlantUML available icons
+- see a preview of the suggested icon in its description
+
+![icons](https://raw.githubusercontent.com/plantuml/plantuml-server/master/docs/WebUI/gifs/auto-completion-icons.gif)
+
+### Themes
+
+- type `!t` to get the suggestion `theme`
+- type `!theme ` to get a list of (local) available PlantUML themes.
+
+![themes](https://raw.githubusercontent.com/plantuml/plantuml-server/master/docs/WebUI/gifs/auto-completion-themes.gif)
+
+
+## Validation
+
+### `@start...` and `@end...`
+
+- `@start...` should always be the first command
+- `@end...` should alway be the last command
+- `@start...` should only exists once
+- `@end...` should only exists once
+- `@end...` should have the same type as `@start...`  
+  e.g.: `@startjson ... @endjson`
+
+![start-end](https://raw.githubusercontent.com/plantuml/plantuml-server/master/docs/WebUI/gifs/validation-start-end.gif)