Update HttpHeaders.Add exception docs (#10493)

* Update `HttpHeaders.Add` exception docs

* Update HttpHeaders.xml

Author: MihaZupan

xml/System.Net.Http.Headers/HttpHeaders.xml

@@ -151,6 +151,8 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
+ Header names are enforced to be valid HTTP tokens, where a token is defined as any set of ASCII letters, digits, or symbols from the ``"!#$%&'*+-.^_`|~"`` set, matching [RFC9110](https://www.rfc-editor.org/rfc/rfc9110.html#name-collected-abnf)'s definition. Non-ASCII characters are not allowed in a header name.  
+
  The header `values` will be parsed and validated.  
   
  If the specified header does not exist, the <xref:System.Net.Http.Headers.HttpHeaders.Add%2A> method inserts a new header into the list of header name/value pairs.  
@@ -166,7 +168,7 @@
 
 -or-
 
-New line characters in header values must be followed by a white-space character. 
+The header value format is invalid for this header name, or the header does not support multiple values.
         </exception>
       </Docs>
     </Member>
@@ -220,7 +222,9 @@ New line characters in header values must be followed by a white-space character
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The header `value` will be parsed and validated.  
+ Header names are enforced to be valid HTTP tokens, where a token is defined as any set of ASCII letters, digits, or symbols from the ``"!#$%&'*+-.^_`|~"`` set, matching [RFC9110](https://www.rfc-editor.org/rfc/rfc9110.html#name-collected-abnf)'s definition. Non-ASCII characters are not allowed in a header name.  
+
+ The header `values` will be parsed and validated.  
   
  If the specified header does not exist, the <xref:System.Net.Http.Headers.HttpHeaders.Add%2A> method inserts a new header into the list of header name/value pairs.  
   
@@ -234,7 +238,7 @@ New line characters in header values must be followed by a white-space character
 
 -or-
 
-New line characters in header values must be followed by a white-space character. 
+The header value format is invalid for this header name, or the header does not support multiple values.
         </exception>
       </Docs>
     </Member>