add markup diagnostic message spec

Author: Maria Solano

_specifications/lsp/3.18/general/initialize.md

@@ -885,6 +885,28 @@ interface ServerCapabilities {
 	 */
 	inlineCompletionProvider?: boolean | InlineCompletionOptions;
 
+	/**
+	 * Text document specific server capabilities.
+	 * 
+	 * @since 3.18.0
+	 */
+	textDocument?: {
+		/**
+		 * Capabilities specific to the diagnostic pull model.
+		 * 
+		 * @since 3.18.0
+		 */
+		diagnostic?: {
+			/**
+			 * Whether the server supports `MarkupContent` in diagnostic messages.
+			 * 
+			 * @since 3.18.0
+			 * @proposed
+			 */
+			markupMessageSupport?: boolean;
+		};
+	};
+
 	/**
 	 * Workspace specific server capabilities
 	 */

_specifications/lsp/3.18/language/codeAction.md

@@ -361,6 +361,9 @@ export interface CodeActionContext {
 	 * for the given range. There is no guarantee that these accurately reflect
 	 * the error state of the resource. The primary parameter
 	 * to compute code actions is the provided range.
+	 * 
+	 * Note that the client should check the `textDocument.diagnostic.markupMessageSupport`
+	 * server capability before sending diagnostics with markup messages to a server.
 	 */
 	diagnostics: Diagnostic[];
 

_specifications/lsp/3.18/language/pullDiagnostics.md

@@ -30,6 +30,14 @@ export interface DiagnosticClientCapabilities {
 	 * pulls.
 	 */
 	relatedDocumentSupport?: boolean;
+
+	/**
+	 * Whether the client supports `MarkupContent` in diagnostic messages.
+	 * 
+	 * @since 3.18.0
+	 * @proposed
+	 */
+	markupMessageSupport?: boolean;
 }
 ```
 

_specifications/lsp/3.18/types/diagnostic.md

@@ -1,5 +1,9 @@
 #### <a href="#diagnostic" name="diagnostic" class="anchor"> Diagnostic </a>
 
+- New in version 3.18: support for markup content in diagnostic messages. The support is guarded by the 
+client capability `textDocument.diagnostic.markupMessageSupport`. If a client doesn't signal the capability, 
+servers shouldn't send `MarkupContent` diagnostic messages back to the client.
+
 Represents a diagnostic, such as a compiler error or warning. Diagnostic objects are only valid in the scope of a resource.
 
 ```typescript
@@ -35,8 +39,11 @@ export interface Diagnostic {
 
 	/**
 	 * The diagnostic's message.
+	 * 
+	 * @since 3.18.0 - support for MarkupContent. This is guarded by the client
+	 * capability `textDocument.diagnostic.markupMessageSupport`.
 	 */
-	message: string;
+	message: string | MarkupContent;
 
 	/**
 	 * Additional metadata about the diagnostic.