Update HttpHeaders.Add exception docs

Author: MihaZupan

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

@@ -138,11 +138,11 @@
         <exception cref="T:System.ArgumentException">The name cannot be null or empty.</exception>
         <exception cref="T:System.ArgumentNullException">The values cannot be null or empty.</exception>
         <exception cref="T:System.InvalidOperationException">Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.</exception>
-        <exception cref="T:System.FormatException">The header name format is invalid.
+        <exception cref="T:System.FormatException">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.
 
 -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>
@@ -197,11 +197,11 @@ New line characters in header values must be followed by a white-space character
         </remarks>
         <exception cref="T:System.ArgumentException">The name cannot be null or empty.</exception>
         <exception cref="T:System.InvalidOperationException">Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.</exception>
-        <exception cref="T:System.FormatException">The header name format is invalid.
+        <exception cref="T:System.FormatException">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.
 
 -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>