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-term/src/handlers/completion/path.rs

@@ -113,7 +113,7 @@ pub(crate) fn path_completion(
                     kind: Cow::Borrowed(kind),
                     label: file_name.into(),
                     transaction,
-                    documentation,
+                    documentation: Some(documentation),
                     provider: CompletionProvider::Path,
                 }))
             })

helix-term/src/ui/completion.rs

@@ -523,7 +523,10 @@ impl Component for Completion {
                 None => return,
             },
             CompletionItem::Other(option) => {
-                markdowned(language, None, Some(&option.documentation))
+                let Some(doc) = option.documentation.as_deref() else {
+                    return;
+                };
+                markdowned(language, None, Some(doc))
             }
         };