doc: improve re.ReplaceAllString documentation related to Expand part

[eduardbme]

Update documentation related to #40329 issue

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[eduardbme]

@googlebot I signed it!

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Message from Ian Lance Taylor:

Patch Set 1:

(3 comments)

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Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
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patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
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reviewed as part of the next development cycle. See https://golang.org/s/release
for more details.

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[eduardbme]

Rebased to the current master, can we re-open and land it ?

[heschi]

GitHub says I can't: "The branch was force-pushed or recreated."

[eduardbme]

Okay, created a new one #56507