html/template: document that comments are stripped from html templates

The exsisting documentaion doesn't document that the comments are
stripped from html templates after they are parsed. This patch tries
to document the behavior and provide a rationale along with a workaround
for that.

Fixes #28628

Author: hellozee

src/html/template/template.go

@@ -173,7 +173,10 @@ func (t *Template) DefinedTemplates() string {
 // Parse parses text as a template body for t.
 // Named template definitions ({{define ...}} or {{block ...}} statements) in text
 // define additional templates associated with t and are removed from the
-// definition of t itself.
+// definition of t itself. Comments in the html format i.e <!-- comment -->
+// are stripped while parsing, this is because they make the injection context
+// ambiguous, and so correct escaping can't be guaranteed. A possible workaround
+// is using a template.HTML variable.
 //
 // Templates can be redefined in successive calls to Parse,
 // before the first use of Execute on t or any associated template.