text/template: document newlines in action delimeters

[benjaminjkraft]

The Go 1.16 release notes say

Newlines characters are now allowed inside action delimiters, permitting actions to span multiple lines.

But the text/template documentation says

Except for raw strings, actions may not span newlines, although comments can.

It looks like the latter needs an update for #29770?

[gopherbot]

Change https://go.dev/cl/394676 mentions this issue: text/template: document newlines are allowed in actions