tweak messages, add link to details that will be included on future docs site

Author: Rich-Harris

packages/svelte/messages/compile-errors/template.md

@@ -344,11 +344,15 @@ HTML restricts where certain elements can appear. In case of a violation the bro
 
 ## svelte_options_invalid_tagname
 
-> Tag name must be two or more words joined by the "-" character
+> Tag name must be lowercase and hyphenated
+
+See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name for more information on valid tag names
 
 ## svelte_options_reserved_tagname
 
-> Tag name is reserved and conflicts with standard element names
+> Tag name is reserved
+
+See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name for more information on valid tag names
 
 ## svelte_options_unknown_attribute
 

packages/svelte/src/compiler/errors.js

@@ -1352,21 +1352,21 @@ export function svelte_options_invalid_customelement_shadow(node) {
 }
 
 /**
- * Tag name must be two or more words joined by the "-" character
+ * Tag name must be lowercase and hyphenated
  * @param {null | number | NodeLike} node
  * @returns {never}
  */
 export function svelte_options_invalid_tagname(node) {
-	e(node, "svelte_options_invalid_tagname", "Tag name must be two or more words joined by the \"-\" character");
+	e(node, "svelte_options_invalid_tagname", "Tag name must be lowercase and hyphenated");
 }
 
 /**
- * Tag name is reserved and conflicts with standard element names
+ * Tag name is reserved
  * @param {null | number | NodeLike} node
  * @returns {never}
  */
 export function svelte_options_reserved_tagname(node) {
-	e(node, "svelte_options_reserved_tagname", "Tag name is reserved and conflicts with standard element names");
+	e(node, "svelte_options_reserved_tagname", "Tag name is reserved");
 }
 
 /**

packages/svelte/tests/validator/samples/tag-invalid/errors.json

@@ -1,7 +1,7 @@
 [
 	{
 		"code": "svelte_options_invalid_tagname",
-		"message": "Tag name must be two or more words joined by the \"-\" character",
+		"message": "Tag name must be lowercase and hyphenated",
 		"start": {
 			"line": 1,
 			"column": 16

packages/svelte/tests/validator/samples/tag-reserved/errors.json

@@ -1,7 +1,7 @@
 [
 	{
 		"code": "svelte_options_reserved_tagname",
-		"message": "Tag name is reserved and conflicts with standard element names",
+		"message": "Tag name is reserved",
 		"start": {
 			"line": 1,
 			"column": 16