builtin: update new documentation for Go 1.26 expression syntax [1.26 backport]

[gopherbot]

@ianlancetaylor requested issue #77584 to be considered for backport to the next 1.26 minor release.

@gopherbot Please open backport to 1.26.

This is a documentation omission that we should fix in the 1.26 minor releases.

[mehrdadbn9]

hi maintainer

if i could help please let me do it and assign it to me

[mehrdadbn9]

I'd like to work on this backport. I can update the new builtin documentation for Go 1.26 expression syntax in the release branch. Please assign if appropriate.

[dmitshur]

Please see https://go.dev/wiki/MinorReleases for more information on the backport process. This cherry pick candidate is waiting to be reviewed first. We'll ask for your help with creating the backport CL after the review happens. Thanks.