Sections in documentation need to become up-to-date

[mturilli]

Current documentation was not able to clarify the questions listed at: radical-collaboration/facts#23

Further, examples at https://radicalentk.readthedocs.io/en/latest/user_guide/adding_shared_data.html do not clarify the use of copy_output_data (> notation) and the different between copy_output_data and download_output_data.

[lee212]

Changed the title to reflect the following items as well:

• best practice on multiple tasks: Adding a modified task to a stage failed radical-collaboration/hpc-workflows#118
• description on task.post_exec method according to facts#23
• shared_data API behavior according to hpc-workflows#119