clock: don't recommend NewTimer

[huww98]

What type of PR is this?

/kind documentation

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind test
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

The GC issue is already resolved in Go 1.23. Copying the comments from time.After() to explain this.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Release note:

NONE

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[skitt]

With the change in Go 1.23, this information isn’t particularly useful for callers of After; it’s useful for callers of NewTimer who may want to reconsider and use After instead.

[huww98]

These are just copied from the stdlib. I think this can be useful for people who are still using Go older than 1.23, as the comments implies there are GC issues before 1.23.