Add comment about BaseAddress merging (#4721)

* Add comment about BaseAddress merging

* Update xml/System.Net.Http/HttpClient.xml

Co-authored-by: Marie Píchová <11718369+ManickaP@users.noreply.github.com>

Co-authored-by: Jan Jahoda <jajahoda@.microsoft.com>
Co-authored-by: Marie Píchová <11718369+ManickaP@users.noreply.github.com>

Author: 3 people

xml/System.Net.Http/HttpClient.xml

@@ -346,6 +346,8 @@ The specified `handler` will be disposed of by calling [HttpClient.Dispose](xref
   
 ## Remarks  
  When sending a <xref:System.Net.Http.HttpRequestMessage> with a relative Uri, the message Uri will be added to the <xref:System.Net.Http.HttpClient.BaseAddress%2A> property to create an absolute Uri.  
+ 
+ Note that all characters after the right-most "/" in the base URI are excluded when combined with the message URI. See [RFC 3986](https://tools.ietf.org/html/rfc3986) Uniform Resource Identifier (URI) Generic Syntax specification.
   
  ]]></format>
         </remarks>