[ty] add docstrings to completions based on type (#20008)

This is a fairly simple but effective way to add docstrings to like 95%
of completions from initial experimentation.

Fixes https://github.com/astral-sh/ty/issues/1036

Although ironically this approach *does not* work specifically for
`print` and I haven't looked into why.

crates/ty_server/src/server/api/requests/completion.rs

@@ -2,7 +2,9 @@ use std::borrow::Cow;
 use std::time::Instant;
 
 use lsp_types::request::Completion;
-use lsp_types::{CompletionItem, CompletionItemKind, CompletionParams, CompletionResponse, Url};
+use lsp_types::{
+    CompletionItem, CompletionItemKind, CompletionParams, CompletionResponse, Documentation, Url,
+};
 use ruff_db::source::{line_index, source_text};
 use ty_ide::completion;
 use ty_project::ProjectDatabase;
@@ -64,9 +66,12 @@ impl BackgroundDocumentRequestHandler for CompletionRequestHandler {
             .map(|(i, comp)| {
                 let kind = comp.kind(db).map(ty_kind_to_lsp_kind);
                 CompletionItem {
-                    label: comp.name.into(),
+                    label: comp.inner.name.into(),
                     kind,
                     sort_text: Some(format!("{i:-max_index_len$}")),
+                    documentation: comp
+                        .documentation
+                        .map(|docstring| Documentation::String(docstring.render_plaintext())),
                     ..Default::default()
                 }
             })