[SQL] Documentation: Explain cacheTable command

add the `cacheTable` specification

Author: CrazyJvm <crazyjvm@gmail.com>

Closes #1681 from CrazyJvm/sql-programming-guide-cache and squashes the following commits:

0a231e0 [CrazyJvm] grammar fixes
a04020e [CrazyJvm] modify title to Cached tables
18b6594 [CrazyJvm] fix format
2cbbf58 [CrazyJvm] add cacheTable guide

docs/sql-programming-guide.md

@@ -769,3 +769,13 @@ To start the Spark SQL CLI, run the following in the Spark directory:
 Configuration of Hive is done by placing your `hive-site.xml` file in `conf/`.
 You may run `./bin/spark-sql --help` for a complete list of all available
 options.
+
+# Cached tables
+
+Spark SQL can cache tables using an in-memory columnar format by calling `cacheTable("tableName")`.
+Then Spark SQL will scan only required columns and will automatically tune compression to minimize
+memory usage and GC pressure. You can call `uncacheTable("tableName")` to remove the table from memory.
+
+Note that if you just call `cache` rather than `cacheTable`, tables will _not_ be cached in
+in-memory columnar format. So we strongly recommend using `cacheTable` whenever you want to
+cache tables.