Improve the documentation of Figure.shift_origin()

[seisman]

Description of proposed changes

• Fix a broken link
• Add a note that the function can't be used as the fisrt plotting command.

Address #514.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.