docs: gno does not support shadowing native types (#1711)

closes #1091 

This is a simple documentation update to clarify that gno does not
support shadowing native types. I've discussed with @jaekwon and this is
not something we want to support.

---------

Co-authored-by: Leon Hudak <33522493+leohhhn@users.noreply.github.com>

docs/reference/go-gno-compatibility.md

@@ -4,7 +4,7 @@ id: go-gno-compatibility
 
 # Go - Gno compatibility
 
-## Native keywords
+## Reserved keywords
 
 | keyword     | support                |
 |-------------|------------------------|
@@ -34,7 +34,14 @@ id: go-gno-compatibility
 
 Generics are currently not implemented.
 
-## Native types
+Note that Gno does not support shadowing of built-in types. 
+While the following built-in typecasting assignment would work in Go, this is not supported in Gno.
+
+```go
+rune := rune('a')
+```
+
+## Builtin types
 
 | type                                          | usage                  | persistency                                                |
 |-----------------------------------------------|------------------------|------------------------------------------------------------|
@@ -57,7 +64,7 @@ Generics are currently not implemented.
 
 **\*:** depends on `T`/`T1`/`T2`
 
-Additional native types:
+Additional builtin types:
 
 | type     | comment                                                                                    |
 |----------|--------------------------------------------------------------------------------------------|

gnovm/pkg/gnolang/preprocess.go

@@ -3282,7 +3282,7 @@ func fillNameExprPath(last BlockNode, nx *NameExpr, isDefineLHS bool) {
 				} else {
 					path = last.GetPathForName(nil, nx.Name)
 					if path.Type != VPBlock {
-						panic("expected block value path type")
+						panic("expected block value path type; check this is not shadowing a builtin type")
 					}
 					break
 				}