minor docs improvements

[kmaterna]

First ever pull request! I've been using pygmt and it's great- thanks for making it.
I found a few minor points in the docs that would hopefully clarify "coast" and "-B" in my experience.

[welcome]

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[weiji14]

Hi @kmaterna, thanks for opening this Pull Request! I just have one minor suggestion (add an alias for L in coast), otherwise it looks good. Let me know If there's any other related improvements you'd like to add, and I'll wait for those before merging this PR in 😄

[welcome]

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[weiji14]

Thanks @kmaterna! Feel free to point out any other things in the documentation you'd like to improve, or new features you'd like to see in the future. Pull requests like this are always very welcome 😃