Better way to document the aliases in docstrings

In the initial version of the alias system, each GMT option flag is aliased to a single parameter PyGMT, so documenting the aliases in docstrings is simple and can be fully automatic. The aliases docstrings are like:
```
**Aliases:**

- J=projection
- B=frame
- R=region
```
Then we realized that sometimes an option flag is too complicated and should be aliased to multiple parameters, and they're documented like below since #3965:
```
**Aliases:**

- J=projection
- B=frame
- R=region
- A=constantfill/gridfill/neighborfill/splinefill
```

With the new alias system introduced in #4000, we have to manually maintain the aliases in docstrings.

We need to discuss what the docstrings look like, especially for option flags that are aliased to multiple parameters. Let me take `-U` as an example. The GMT CLI syntax is `-U[label][+jjustify][+odx[/dy]][+ttext]`, then the docstrings can be:

**U**=label, justify (**+j**), offset (**+o**), text (**+t**)

or

**U**=label, **+j**: justify, **+o**: offset, **+t**: text

We also need to add a documentation explaining how the new alias system works and how to read and understand the aliases in docstrings.

_Originally posted by @seisman in https://github.com/GenericMappingTools/pygmt/issues/4000#issuecomment-3111800039_