DOC: added a reference to DataFrame assign in concatenate section of merging
Because of various stack overflow answers we are directed to the merging
page where a link to DataFrame.assign would be better. Adding the
reference here will make people's code better. At least it would have
for me.

- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

For tests, we'll see with CI.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18665?src=pr&el=h1) Report
> Merging [#18665](https://codecov.io/gh/pandas-dev/pandas/pull/18665?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/740ad9aabda113e34de0d30c78b8aec40414daf7?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18665/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18665?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18665      +/-   ##
==========================================
- Coverage   91.61%    91.6%   -0.01%     
==========================================
  Files         150      150              
  Lines       48887    48864      -23     
==========================================
- Hits        44786    44763      -23     
  Misses       4101     4101
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.77% <ø> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18665?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/18665/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.62% <0%> (-0.13%)` | :arrow_down: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/18665/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.3% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18665?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18665?src=pr&el=footer). Last update [740ad9a...22c1fcf](https://codecov.io/gh/pandas-dev/pandas/pull/18665?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I made the requested changes. can you rebase Rebase done. Updated changelog in version 0.23.0 as per milestone attributed. Rewritten the contribution to be a simple note pointer to the `DataFrame.assign()` function. I noticed that in the meantime the `assign()` documentation was bonified and includes a column assignment using values derived from the dataframe. This is exactly what I hoped I'd found when I was looking for it in the `merging` documentation. Updated text to reflect @TomAugspurger's suggestion based on @jreback's comments. thanks @obilodeau 