[DOCS] Querying basics: what can be graphed

Put a scalar to query, it can be graphed.

So the doc says "an expression that returns an instant vector is the only type which can be graphed." is not correct?

And also, a query_range, which used for graph, always return a range vector <https://promlabs.com/blog/2020/06/18/the-anatomy-of-a-promql-query/#range-queries> , so it's confusing to read the above statement.

Signed-off-by: Viet Hung Nguyen <hvn@familug.org>

docs/querying/basics.md

@@ -35,8 +35,9 @@ evaluate to one of four types:
 
 Depending on the use-case (e.g. when graphing vs. displaying the output of an
 expression), only some of these types are legal as the result of a
-user-specified expression. For example, an expression that returns an instant
-vector is the only type which can be graphed.
+user-specified expression. 
+For [instant queries](api.md#instant-queries), any of the above data types are allowed as the root of the expression.
+[Range queries](api.md/#range-queries) only support scalar-typed and instant-vector-typed expressions.
 
 _Notes about the experimental native histograms:_