core: Make completion item documentation optional

Path completion items always have documentation but future core (i.e.
non-LSP) completions may not always have documentation - for example
word completion from the current buffer.

helix-core/src/completion.rs

@@ -8,7 +8,7 @@ pub struct CompletionItem {
     pub label: Cow<'static, str>,
     pub kind: Cow<'static, str>,
     /// Containing Markdown
-    pub documentation: String,
+    pub documentation: Option<String>,
     pub provider: CompletionProvider,
 }