DOC: Remove incorrect elements of PeriodIndex docstring
 - Closes #9056


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=h1) Report
> Merging [#16553](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/03d44f3dd0ffd55d7538b67466cf4d3899ceac27?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16553/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16553   +/-   ##
=======================================
  Coverage   90.75%   90.75%           
=======================================
  Files         161      161           
  Lines       51074    51074           
=======================================
  Hits        46353    46353           
  Misses       4721     4721
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.59% <ø> (ø)` | :arrow_up: |
| #single | `40.16% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.74% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16553?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/16553?src=pr&el=footer). Last update [03d44f3...1e78866](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=h1) Report
> Merging [#16553](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/03d44f3dd0ffd55d7538b67466cf4d3899ceac27?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16553/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16553   +/-   ##
=======================================
  Coverage   90.75%   90.75%           
=======================================
  Files         161      161           
  Lines       51074    51074           
=======================================
  Hits        46353    46353           
  Misses       4721     4721
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.59% <ø> (ø)` | :arrow_up: |
| #single | `40.16% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.74% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16553?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/16553?src=pr&el=footer). Last update [03d44f3...51c329b](https://codecov.io/gh/pandas-dev/pandas/pull/16553?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Removed a trailing space. Thanks @kiwirob !