### What changes were proposed in this pull request?

Link to appropriate Java Class with list of date/time patterns supported

### Why are the changes needed?

Avoid confusion on the end-user's side of things, as seen in questions like [this](https://stackoverflow.com/questions/54496878/date-format-conversion-is-adding-1-year-to-the-border-dates) on StackOverflow

### Does this PR introduce any user-facing change?

Yes, Docs are updated.

### How was this patch tested?

`date_format`:
![image](https://user-images.githubusercontent.com/2394761/70796647-b5c55900-1d9a-11ea-89f9-7a8661641c09.png)

`to_unix_timestamp`:
![image](https://user-images.githubusercontent.com/2394761/70796664-c07fee00-1d9a-11ea-9029-e82d899e3f59.png)

`unix_timestamp`:
![image](https://user-images.githubusercontent.com/2394761/70796688-caa1ec80-1d9a-11ea-8868-a18c437a5d49.png)

`from_unixtime`:
![image](https://user-images.githubusercontent.com/2394761/70796703-d4c3eb00-1d9a-11ea-85fe-3c672e0cda28.png)

`to_date`:
![image](https://user-images.githubusercontent.com/2394761/70796718-dd1c2600-1d9a-11ea-81f4-a0966eeb0f1d.png)

`to_timestamp`:
![image](https://user-images.githubusercontent.com/2394761/70796735-e6a58e00-1d9a-11ea-8ef7-d3e1d9b5370f.png)

Closes #26864 from johnhany97/SPARK-30236.

Authored-by: John Ayad <johnhany97@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>