`schema_from_json` docs and samples

[tswast]

The schema_from_json method is useful for loading a BigQuery schema file into the Python objects expected by our client library. There are several opportunities for Python code samples in the Using a BigQuery JSON schema file section of the docs describing that format:

• You can write an existing table schema to a local file by entering the following command [Python code] (schema_to_json sample) -- bigquery_schema_file_from_table
• The following command [Python code] loads data into a table using the schema definition in a JSON file -- bigquery_load_with_schema_file
• The following command [Python code] creates an empty table in an existing dataset using the schema definition in a JSON file -- bigquery_create_table_with_schema_file

[tswast]

In addition, I think schema_from_json is missing a return type based on what I'm seeing in the docs.

[tswast]

@loferris Potential task for once you've finished the code samples at #778