Remove the inconsistency in table/function name conventions in SparkSession.Catalog APIs

Observed by @felixcheung , in `SparkSession`.`Catalog` APIs, we have different conventions/rules for table/function identifiers/names. Most APIs accept the qualified name (i.e., `databaseName`.`tableName` or `databaseName`.`functionName`). However, the following five APIs do not accept it.

• def listColumns(tableName: String): Dataset[Column]
• def getTable(tableName: String): Table
• def getFunction(functionName: String): Function
• def tableExists(tableName: String): Boolean
• def functionExists(functionName: String): Boolean

It is desirable to make them more consistent with the other Catalog APIs, updates the function/API comments and adds the `@params` to clarify the inputs we allow.