Issue 59 update documentation how to use send back diagram image

[CopyDemon]

Fixes

1. export_flowsheet_diagram function missing param display
2. add documentation for function export_flowsheet_diagram

Proposed changes:
Missing documentation for new function export_flowsheet_diagram

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[CopyDemon]

@dangunter please help me review this PR. The documentation is at User Documentation > Save and Preview Flowsheet Diagram