Clarify client responsibility to convert selection into a position.

Author: dbaeumer

_specifications/specification-3-16.md

@@ -936,7 +936,7 @@ interface VersionedTextDocumentIdentifier extends TextDocumentIdentifier {
 
 Was `TextDocumentPosition` in 1.0 with inlined parameters.
 
-A parameter literal used in requests to pass a text document and a position inside that document.
+A parameter literal used in requests to pass a text document and a position inside that document. It is up to the client to decide how a selection is converted into a position when issuing a request for a text document. The client can for example honor or ignore the selection direction to make LSP request consistent with features implemented internally.
 
 ```typescript
 interface TextDocumentPositionParams {