Consistent bullet point lists in docstrings

[yvonnefroehlich]

Let's take the documentation of Figure.plot (https://www.pygmt.org/dev/api/generated/pygmt.Figure.plot.html#pygmt.Figure.plot) as an example. Currently, we use different ways to write bullet point lists in the docstrings:

• modifier: docstring (e.g., connection)
• modifier - docstring (e.g., gap)
• modifier docstring (e.g., header)

I feel it would be good to decide for one way and then update all docstrings accordingly.

[seisman]

Yes, I agree. Personally, I prefer using :.

[yvonnefroehlich]

Yes, I agree. Personally, I prefer using :.

@GenericMappingTools/pygmt-maintainers are there any other preferences? Otherwise we will go with :.

[weiji14]

Yep, ok with using :.