Elixir: inject Markdown into docs, remove h sigil HEEx injection (#2619)

Co-authored-by: Michael Davis <mcarsondavis@gmail.com>

runtime/queries/elixir/injections.scm

@@ -11,6 +11,15 @@
 ((sigil
   (sigil_name) @_sigil_name
   (quoted_content) @injection.content)
- (#match? @_sigil_name "^(h|H)$")
+ (#eq? @_sigil_name "H")
  (#set! injection.language "heex")
  (#set! injection.combined))
+
+(unary_operator
+  operator: "@"
+  operand: (call
+  target: ((identifier) @_identifier (#match? @_identifier "^(module|type|short)?doc$"))
+    (arguments [
+      (string (quoted_content) @injection.content)
+      (sigil (quoted_content) @injection.content)
+  ])) (#set! injection.language "markdown"))