streams: implement readStr for VM, document VM limitations (#24058)

fixes #24054

`readData` is not implemented for the VM as mentioned in the issue, but
`readDataStr` is, so that is used for `readStr` instead on the VM. We
could also just use it in general since it falls back to `readData`
anyway but it's kept the same otherwise for now.

Also where and why streams in general don't work in VM is now documented
on the top level `streams` module documentation.

tests/stdlib/tstreams.nim

@@ -92,6 +92,10 @@ static: # Ensure streams it doesnt break with nimscript on arc/orc #19716
   let s = newStringStream("a")
   doAssert s.data == "a"
 
+static: # issue #24054, readStr
+  var s = newStringStream("foo bar baz")
+  doAssert s.readStr(3) == "foo"
+
 template main =
   var strm = newStringStream("abcde")
   var buffer = "12345"