[SPARK-28833][DOCS][SQL] Document ALTER VIEW command
[SPARK-28833][DOCS][SQL] Document ALTER VIEW command

### What changes were proposed in this pull request?
Document ALTER VIEW statement in the SQL Reference Guide.

### Why are the changes needed?
Currently Spark SQL doc lacks documentation on the supported SQL syntax. This pr is aimed to address this issue.

### Does this PR introduce any user-facing change?
Yes
#### Before:
There was no documentation for this.

#### After:
![Screen Shot 2019-11-13 at 10 51 33 PM](https://user-images.githubusercontent.com/7550280/68833575-ac947f80-0668-11ea-910f-c133407ef502.png)
![Screen Shot 2019-11-13 at 10 56 42 PM](https://user-images.githubusercontent.com/7550280/68833597-bae29b80-0668-11ea-9782-b7be94789c12.png)
![Screen Shot 2019-11-13 at 10 56 53 PM](https://user-images.githubusercontent.com/7550280/68833607-be762280-0668-11ea-8a30-5602e755bab8.png)

### How was this patch tested?
Tested using jkyll build --serve

Closes #25573 from kevinyu98/spark-28833-alterview.

Authored-by: Kevin Yu <58f74537bdbc2dd7be823680c4a7e30cfa52e668@us.ibm.com>
Signed-off-by: Sean Owen <3f9d48d7c3f24f7009479c5fa630bd1131ffc724@databricks.com>

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