Document union_find_replacet

[romainbrenguier]

In particular, the documentation for make_union was outdated.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[allredj]

Passed Diffblue compatibility checks (cbmc commit: 75b8386).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89282695

[smowton]

Does this function provide that guarantee, or require it from the caller?

[kroening]

I think that guarantee is too obscure/non-standard to provide.
To make anything the root, use re_root()

[romainbrenguier]

I removed this part of the comment, because changing the implementation may break this.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: df01f72).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/89637573
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.