Improve comments/descriptions of inline examples for plotting methods

[seisman]

Changes include:

• Ensure each inline example has at least one-sentence description
• Remove some trivial comments like Create a new plot with pygmt.Figure(), Show the plot
• Rewrap a few comments

[yvonnefroehlich]

I would make the headings consistent and use either plural or singular (independent of the number of examples). I prefer plural, i.e. "Examples", as we use plural also for the heading for the gallery examples below.

[seisman]

I would make the headings consistent and use either plural or singular (independent of the number of examples). I prefer plural, i.e. "Examples", as we us plural also for the heading for the gallery examples below.

That's a good point. I'll address it in a separate PR and also update our "Style Checks" workflow to enforce the style.