DOC: Updated the docstring of Series.rank / DataFrame.rank
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I have updated the docstring following the convention and I have added some examples.

Please let me know if I can further improve this.

Thanks
Hello @EdAbati! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/EdAbati/pandas/blob/ccd751805d90ad8182b723e4ea3427721ef4b77b/pandas/core/generic.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23263?src=pr&el=h1) Report
> Merging [#23263](https://codecov.io/gh/pandas-dev/pandas/pull/23263?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b4b945adad587851d570e950d05a1da75c5dc76c?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/23263/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23263?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23263   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51339    51339           
=======================================
  Hits        47360    47360           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <ø> (ø)` | :arrow_up: |
| #single | `42.34% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23263?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23263/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.82% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23263?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/23263?src=pr&el=footer). Last update [b4b945a...57f566b](https://codecov.io/gh/pandas-dev/pandas/pull/23263?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @EdAbati do you have time to update this PR? ping @datapythonista  Closing as stale, @EdAbati let us know if you want to continue working on it. Hi @datapythonista,

Extremely sorry for the delay.
I have been very busy in the last month or so.
I was planning to come back to this one next when I will be on holiday.
 Hi @datapythonista ,

I gave it another try.
I have opened another pull request where I have made the changes requested above.