Add note about UrlEncode

xml/System.Net/WebUtility.xml

@@ -556,6 +556,9 @@
 
  Each ' ' (space) character is converted to a `+` (plus) character.
 
+> [!NOTE]
+>  Replacing ' ' (space) with a '+' (plus) sign is expected only in query part of URI (by convention, not by standard) or in [`application/x-www-form-urlencoded`](https://url.spec.whatwg.org/#percent-encoded-bytes) HTTP content. Moreover, the above mentioned list of characters that are **not** replaced does not correspond neither to [URL specification](https://url.spec.whatwg.org) nor URI [RFC 3986](https://www.rfc-editor.org/rfc/rfc3986.html#section-3.4). Use <xref:System.Uri.WebUtility.EscapeDataString%2A> for RFC compliant transformation.
+
  For example, when embedded in a block of text to be transmitted in a URL, the characters `<` and `>` are encoded as `%3c` and `%3e`.
 
  The <xref:System.Net.WebUtility.UrlDecode%2A> method reverses the encoding.
@@ -636,6 +639,11 @@
 
 -   `(` and `)` (opening and closing parentheses)
 
+ Each ' ' (space) character is converted to a `+` (plus) character.
+
+> [!NOTE]
+>  Replacing ' ' (space) with a '+' (plus) sign is expected only in query part of URI (by convention, not by standard) or in [`application/x-www-form-urlencoded`](https://url.spec.whatwg.org/#percent-encoded-bytes) HTTP content. Moreover, the above mentioned list of characters that are **not** replaced does not correspond neither to [URL specification](https://url.spec.whatwg.org) nor URI [RFC 3986](https://www.rfc-editor.org/rfc/rfc3986.html#section-3.4). Use <xref:System.Uri.WebUtility.EscapeDataString%2A> for RFC compliant transformation.
+
  For example, when embedded in a block of text to be transmitted in a URL, the characters `<` and `>` are encoded as `%3c` and `%3e`.
 
  The <xref:System.Net.WebUtility.UrlDecodeToBytes%2A> method reverses the encoding.