Improve the description of usage of remote datasets

[seisman]

Take the earth_age dataset as an example. Currently, the documentation says:

These grids can also be accessed by passing in the file name @earth_age_res[_reg] to any grid plotting/processing function. res is the grid resolution (see below), and reg is grid registration type (p for pixel registration or g for gridline registration).

There are some issues with the description:

1. People may have no idea that [] means _reg is optional.
2. Since GMT 6.3.0 (see Implement auto resolution for remote grids for plotting modules gmt#5753), GMT can automatically determine the suitable grid resolution based on the -R and -J settings. Thus _res is also optional.

Here is a working example showing how the automatic grid resolution works for remote datasets:

import pygmt

fig = pygmt.Figure()
fig.basemap(region="g", frame=True, projection="H10c")
fig.grdimage(grid="@earth_relief")
fig.show()

We need to improve the documentation to clarify these two points.