gopls/doc: add two missing items to doc index

Type Hierarchy, and support for Go assembly files.

Change-Id: Ic9cfe0a8672d837707eb65b8b0830efe5d5cc252
Reviewed-on: https://go-review.googlesource.com/c/tools/+/680297
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

Author: adonovan

gopls/doc/features/README.md

@@ -42,6 +42,7 @@ when making significant changes to existing features or when adding new ones.
   - [Symbol](navigation.md#symbol): fuzzy search for symbol by name
   - [Selection Range](navigation.md#selection-range): select enclosing unit of syntax
   - [Call Hierarchy](navigation.md#call-hierarchy): show outgoing/incoming calls to the current function
+  - [Type Hierarchy](navigation.md#type-hierarchy): show interfaces/implementations of the current type
 - [Completion](completion.md): context-aware completion of identifiers, statements
 - [Code transformation](transformation.md): fixes and refactorings
   - [Formatting](transformation.md#formatting): format the source code
@@ -58,6 +59,7 @@ when making significant changes to existing features or when adding new ones.
 - Support for non-Go files:
   - [Template files](templates.md): files parsed by `text/template` and `html/template`
   - [go.mod and go.work files](modfiles.md): Go module and workspace manifests
+  - [Go *.s assembly files](assembly.ms): Go assembly files
 - [Command-line interface](../command-line.md): CLI for debugging and scripting (unstable)
 
 You can find this page from within your editor by executing the

gopls/doc/features/assembly.md

@@ -0,0 +1,29 @@
+# Gopls: Support for Go *.s assembly files
+
+Gopls has rudimentary support for LSP operations in Go assembly files.
+
+Go assembly files have a `.s` file name extension. LSP clients need to
+be configured to recognize `.s` files as Go assembly files, since this
+file name extension is also used for assembly files in other
+languages. A good heuristic is that if a file named `*.s` belongs to a
+directory containing at least one `*.go` file, then the `.s` file is
+Go assembly, and its appropriate language server is gopls.
+
+Only Definition (`textDocument/definition`) requests are currently
+supported. For example, a Definition request on the `sigpanic`
+symbol in this file in GOROOT/src/runtime/asm.s:
+
+```asm
+	JMP	·sigpanic<ABIInternal>(SB)
+```
+
+returns the location of the function declaration in
+GOROOT/src/runtime/signal_unix.go:
+
+```go
+//go:linkname sigpanic
+func sigpanic() {
+```
+
+See also issue golang/go#71754, which tracks the development of LSP
+features in Go assembly files.