Expand pygmt subplot's documentation for the figsize and subsize parameters

[weiji14]

Description of the desired feature

The docstring for pygmt.subplot's figsize (-Ff) and subsize (-Fs) parameters are rather brief, and we should expand it to be longer following upstream GMT at https://docs.generic-mapping-tools.org/6.2/subplot.html#required-arguments-begin-mode. This is a follow up to #822.

Current state 📝

pygmt/pygmt/src/subplot.py

Lines 56 to 61 in d90b3fc

	figsize : tuple
	Specify the final figure dimensions as (*width*, *height*).
	subsize : tuple
	Specify the dimensions of each subplot directly as (*width*, *height*).
	Note that only one of ``figsize`` or ``subsize`` can be provided at
	once.

Desired state ✨

figsize : tuple
    Specify the final figure dimensions as (*width*, *height*). The subplot
    dimensions are then calculated from the figure dimensions after
    accounting for the space that optional tick marks, annotations, labels,
    and margins occupy between subplots. As for other figures, annotations,
    ticks, and labels along the outside perimeter are not counted as part
    of the figure dimensions. To specify different subplot dimensions for
    each row (or column), append **+f** followed by a comma-separated list
    of width fractions, a slash, and then the list of height fractions. For
    example ``figsize="10c/10c+f3,1/1,2"`` will make the first column three
    times as wide as the second, while the second row will be twice as tall
    as the first row. A single number means constant widths (or heights)
    [Default].

subsize : tuple
    Specify the dimensions of each subplot directly as (*width*, *height*).
    Then, the figure dimensions are computed from the subplot dimensions
    after adding the space that optional tick marks, annotations, labels,
    and margins occupy between subplots. As for other figures, annotations,
    ticks, and labels along the outside perimeter are not counted as part
    of the figure dimensions. To specify different subplot dimensions for
    each row (or column), append a comma-separated list of widths, a slash,
    and then the comma-separated list of heights. A single number means
    constant widths (or heights) [Default]. For example
    ``subsize="5c,8c/8c"`` will make the first column 5 cm wide and the
    second column 8 cm wide, with all having a constant height of 8 cm. The
    number of values must either be one (constant across the rows or
    columns) or exactly match the number of rows (or columns). For
    geographic maps, the height of each subplot depends on your map region
    and projection. There are two options:

    1. Specify both ``region`` and ``projection`` and we use these to
       compute the height of each subplot. All subplots must share the same
       region and projection and you specify a zero *height*, or
    2. Select *height* based on trial and error to suit your plot layout.

    Optionally, you may draw the outline (**+p**\ *pen*) or paint
    (**+g**\ *fill*\) the figure rectangle behind the subplots, add
    dividing lines between panels (**+w**\ *pen*), and even expand it via
    **+c**. These are most useful if you supply **frame="+n"** to
    ``subplot``, meaning no ticks or annotations will take place in the
    subplots. By default (**+af**), we auto-scale the fonts and pens based
    on the geometric mean dimension of the entire figure. Append **+as** to
    instead determine the scaling from the geometric mean subplot
    dimension.

How to make the change 🧐

1. First, read up our contributing guidelines at https://www.pygmt.org/v0.4.0/contributing.html#editing-the-documentation
2. Leave a comment below, stating that you will tackle this issue. We will assign you to this issue and you can then start working on it.
3. To edit the subplot module's docstring, go to https://github.com/GenericMappingTools/pygmt/edit/master/pygmt/src/subplot.py and add in the new docstring!
4. Submit a pull request, and be sure to fill it up with as much detail as possible.

🥳 Good luck, and let us know if you need any help :)

Are you willing to help implement and maintain this feature? Happy to review someone's Pull Request on it.