Spellcheck of docs, a few minor changes
This PR continues my read-through of the docs, the previous PRs submitted are #18941 and  #18948.

The following edits have been made:
- Missing periods and colons added before introducing code examples.
- Increased number of function references (clickable links).
- Cleared up a few sentences which I found unclear.

Feedback is welcome.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18973?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@10edfd0`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18973/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18973?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #18973   +/-   ##
=========================================
  Coverage          ?   91.58%           
=========================================
  Files             ?      150           
  Lines             ?    48972           
  Branches          ?        0           
=========================================
  Hits              ?    44851           
  Misses            ?     4121           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <ø> (?)` | |
| #single | `41.72% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18973?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18973?src=pr&el=footer). Last update [10edfd0...9133861](https://codecov.io/gh/pandas-dev/pandas/pull/18973?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks for the feedback @TomAugspurger. I have submitted a new PR addressing your comments, which were helpful. 

- I agree that "*... to compute it, use `my_method` and ...*" is more succinct and clear than "*... to compute it, use the method `my_method` and ...*".
- I am uncertain about starting a sentence with "*The `my_method` method is...*" vs. "*`my_method` is...*". The first phrasing seems to occur quite a bit in the documentation. @jreback I've addressed your comments. I've been noting some stylistic preferences (which I agree with), such as "*It is ...*" instead of "*It's*" and using double-backticks on objects. For now I have not addressed these globally in the docs, but I hope to do so in a future PR. @tommyod totally cool. updating things like this is generally, do a few docs / files at once. it becomes too hard to review otherwise. @jreback :+1: . Addressed comments. thanks @tommyod 