fix: add missing documentation for connection properties (#155)

Add documentation for properties that are supported for end users.
This change does not add documentation for the userAgent property, as
it is not a property that should be set by end users. This property
should only be set by libraries that are maintained by Google.

Fixes #152

Author: olavloite

src/main/java/com/google/cloud/spanner/jdbc/JdbcDriver.java

@@ -87,6 +87,8 @@
  *       connection. Default is true. @see {@link
  *       com.google.cloud.spanner.jdbc.CloudSpannerJdbcConnection#setRetryAbortsInternally(boolean)}
  *       for more information.
+ *   <li>numChannels (int): Sets the number of gRPC channels to use. Defaults to 4.
+ *   <li>usePlainText (boolean): Sets whether the JDBC connection should establish an unencrypted connection to the server. This option can only be used when connecting to a local emulator that does not require an encrypted connection, and that does not require authentication.
  *   <li>optimizerVersion (string): The query optimizer version to use for the connection. The value must be either a valid version number or <code>LATEST</code>. If no value is specified, the query optimizer version specified in the environment variable <code>SPANNER_OPTIMIZER_VERSION<code> will be used. If no query optimizer version is specified in the connection URL or in the environment variable, the default query optimizer version of Cloud Spanner will be used.
  * </ul>
  */