DOC: add concatenation-section to text.rst
As requested by @jreback in #20347.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20790?src=pr&el=h1) Report
> Merging [#20790](https://codecov.io/gh/pandas-dev/pandas/pull/20790?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0ae7e9090cee6e10ebb4124a0b7f3b30811ccc53?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/20790/graphs/tree.svg?height=150&src=pr&width=650&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20790?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20790      +/-   ##
==========================================
- Coverage   91.85%   91.84%   -0.01%     
==========================================
  Files         153      153              
  Lines       49308    49308              
==========================================
- Hits        45290    45288       -2     
- Misses       4018     4020       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.89% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20790?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20790/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20790/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.59% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20790?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/20790?src=pr&el=footer). Last update [0ae7e90...4ae9196](https://codecov.io/gh/pandas-dev/pandas/pull/20790?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 cc @jorisvandenbossche @TomAugspurger  Build failure is not due to my commits (which do not touch the code base, just docs) thanks