Pipeline options documentation has typos and does not show how to use user-defined the options.

Link: 

[https://beam.apache.org/documentation/programming-guide/#configuring-pipeline-options](https://beam.apache.org/documentation/programming-guide/#configuring-pipeline-options)

1) Python and Go snippets incorrectly end with an example from Java tab: "Now your pipeline can accept \--myCustomOption=value as a command-line argument."

 2) The page does not show how users can actually access the values of these user-defined options.

cc: [~melap], [~rtnguyen] [~altay]

Imported from Jira [BEAM-7007](https://issues.apache.org/jira/browse/BEAM-7007). Original Jira may contain additional context.
Reported by: tvalentyn.