text/template: remove an incorrect comment and clarify docs for AddParseTree

The comment "If the name is the name of this template, overwrite this template."
is incorrect and should be "is not" instead. This comment is no longer
required once the docs are updated to mention this behaviour instead.

Fixes #34695

Change-Id: I773495b2194d7bb7619b13c1a28cbc76e8f69aac
Reviewed-on: https://go-review.googlesource.com/c/go/+/199139Reviewed-by: Rob Pike <r@golang.org>

src/text/template/template.go

@@ -119,12 +119,12 @@ func (t *Template) copy(c *common) *Template {
 	}
 }
 
-// AddParseTree adds parse tree for template with given name and associates it with t.
-// If the template does not already exist, it will create a new one.
-// If the template does exist, it will be replaced.
+// AddParseTree associates the argument parse tree with the template t, giving
+// it the specified name. If the template has not been defined, this tree becomes
+// its definition. If it has been defined and already has that name, the existing
+// definition is replaced; otherwise a new template is created, defined, and returned.
 func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
 	t.init()
-	// If the name is the name of this template, overwrite this template.
 	nt := t
 	if name != t.name {
 		nt = t.New(name)