How to reproduce original GMT arguments in PyGMT documentation

Description of the desired feature

The GMT documentation has many instances where bold and italicized fonts are mixed, looking something like:

+nargs

The PyGMT documentation has been rather inconsistent with this (see mentions at https://github.com/GenericMappingTools/pygmt/pull/525#discussion_r454414687 and https://github.com/GenericMappingTools/pygmt/pull/620#discussion_r493088431).

What we should do is to standardize how we want +n args to be written in the documentation. This is a bit complicated, but boils down to (at least) two options:

1. r" **+n**\ *args* ". Use a raw-string in Python, allowing us to follow upstream GMT (as per https://github.com/GenericMappingTools/pygmt/pull/620#discussion_r493088431)

2. " **+n**\\ *args* ". Use a double-backslash to avoid the flake8 error W605 invalid escape sequence '\ '.

An alternative would be to find a way to ignore the flake8 error somehow for the ReST docstrings.

Also, should we find a way to put these in code-formatted blocks (i.e. with backticks), as mentioned at https://github.com/GenericMappingTools/pygmt/pull/525/files#r454746311?

Are you willing to help implement and maintain this feature? Somewhat short on time, but will look into it.

References:

• https://meta.stackexchange.com/questions/183610/how-to-combine-bold-and-code-sample-in-markdown

[maxrjones]

Moving forward is seems that the GMT documentation will be going with “[Default is …]” (see https://github.com/GenericMappingTools/gmt/pull/4834).

[seisman]

As pointed out by @weiji14, the PyGMT documentation is inconsistently using italics, bold, and code strings.

I think we can following these conventions:

1. Use code for Python function parameters
2. Use bold and italics for GMT syntax like +nargs, in which bold +n means it should be typed as it is, and italic args should be given a specific value.

This is a good example of what it looks like (https://www.pygmt.org/dev/tutorials/frames.html):