Grdcontour's long alias for G (label_placement) is not properly documented

[weiji14]

Description of the problem

Documentation for grdcontour uses the common alias {G} which should be a custom one for 'label_placement'. But this incorrectly points to the common alias for color (G):

pygmt/pygmt/base_plotting.py

Line 274 in d3e131a

{G}

pygmt/pygmt/helpers/decorators.py

Lines 37 to 40 in d3e131a

	"G": """\
	color : str
	Select color or pattern for filling of symbols or polygons. Default
	is no fill.""",

The fix shouldn't be too hard, just need to change the line {G} above to the full documentation at https://docs.generic-mapping-tools.org/6.1/grdcontour.html#g.

label_placement : str
    ``[d|f|n|l|L|x|X]params``.
    The required argument controls the placement of labels along the quoted lines. Choose among five controlling algorithms:
    ...

Make sure you wrap the docstring to 79 characters as per #384!

System information

Please paste the output of python -c "import pygmt; pygmt.show_versions()":

pygmt: v0.2.0

[seisman]

The -G option is really complicated. Perhaps we can just write a one-sentence description and then link it to the GMT module documentation. It's not perfect, but at least it's better than the current wrong description.

[willschlitzer]

Can we close this issue now that the -G alias has been explained in the linked PR by @seisman ?