DOC: Spellcheck of merging.rst, reshaping.rst and timeseries.rst
This PR is a continuation of my reading through the docs, see #19017, #18973, #18948, #18941.

The changes are:
* In `merging.rst`, `reshaping.rst` and `timeseries.rst`:
  * Function references as links
  * Backticks ` `` ` around Series, DataFrame
  * Minor rephrasing of sentences, spelling, etc
* Replaced some instances of `python` with `Python` and `numpy` with `NumPy` globally in `*.rst` files. I hope to add a lint rule in a future PR.

Comments/reviews are very welcome.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19081?src=pr&el=h1) Report
> Merging [#19081](https://codecov.io/gh/pandas-dev/pandas/pull/19081?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4a8496b2bf02df521b6b7f15843c515545a3d192?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19081/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19081?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19081      +/-   ##
==========================================
- Coverage   91.54%   91.51%   -0.03%     
==========================================
  Files         148      148              
  Lines       48804    48804              
==========================================
- Hits        44676    44664      -12     
- Misses       4128     4140      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.89% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.63% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19081?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19081/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19081?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/19081?src=pr&el=footer). Last update [4a8496b...4f1a867](https://codecov.io/gh/pandas-dev/pandas/pull/19081?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @tommyod keep em coming!

be sure to glance at the built docs to make sure the edits are ok: http://pandas-docs.github.io/pandas-docs-travis/

built by the CI so can take some time