Document Unicode encoding behavior in Uri.GetLeftPart method (#11479)

Author: Copilot

xml/System/Uri.xml

@@ -2442,6 +2442,12 @@ If you used an escaped string to construct this instance (for example, `"http://
 
 The <xref:System.Uri.GetLeftPart%2A> method returns a string containing the leftmost portion of the URI string, ending with the portion specified by `part`.
 
+> [!IMPORTANT]
+> The <xref:System.Uri.GetLeftPart%2A> method performs Unicode character encoding and normalization as part of its processing. It is not a simple string manipulation method. The returned string may differ from the original URI string due to this encoding behavior.
+
+The <xref:System.Uri.GetLeftPart%2A> method is equivalent to calling <xref:System.Uri.GetComponents%2A> with the appropriate <xref:System.UriComponents> flags. For example:
+- `GetLeftPart(UriPartial.Authority)` is equivalent to `GetComponents(UriComponents.Scheme | UriComponents.UserInfo | UriComponents.Host | UriComponents.Port, UriFormat.UriEscaped)`
+
 <xref:System.Uri.GetLeftPart%2A> includes delimiters in the following cases:
 
 - <xref:System.UriPartial.Scheme> includes the scheme delimiter.