Improve invalid doc expression error message.

Display the unexpanded form of the expression instead of its expansion.

base/docs/Docs.jl

@@ -439,6 +439,12 @@ __doc__!(meta, def) = false
 fexpr(ex) = isexpr(ex, :function, :stagedfunction, :(=)) && isexpr(ex.args[1], :call)
 
 function docm(meta, def, define = true)
+
+    err = (
+        "Invalid doc expression:", def, isexpr(def, :macrocall) ?
+        "'$(def.args[1])' is not documentable. See 'help?> Docs.@__doc__' for details." : ""
+    )
+
     def′ = unblock(def)
 
     isexpr(def′, :quote) && isexpr(def′.args[1], :macrocall) &&
@@ -463,7 +469,7 @@ function docm(meta, def, define = true)
     isvar(def′)                ? objdoc(meta, def′) :
     isexpr(def′, :tuple)       ? multidoc(meta, def′.args) :
     __doc__!(meta, def′)       ? esc(def′) :
-    isa(def′, Expr)            ? error("Invalid doc expression $def′") :
+    isa(def′, Expr)            ? error(strip(join(err, "\n\n"))) :
     objdoc(meta, def′)
 end