text/template: doc: `define` action is not listed and described in "Actions" section

[kidlj]

What version of Go are you using (go version)?

$ go version
go version go1.16.3 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What did you expect to see?

In text/template doc page, define action should be officially listed in 'Actions' section.

What did you see instead?

In text/template doc page, define action is described in a separate section, named "Nested template definitions", however, as an action, it's not listed in the "Actions" section as other actions do. It should also be there.

[cherrymui]

cc @robpike @mvdan