legend: clearances between legend items and the box border

**Description of the problem**

In the **legend** module, it seems the **-F+c** and **-C** options set the clearance between legend terms and the frame. Is it right? If yes, perhaps we should remove the **-C** option from the documentation (still keep it for backward compatibility).

Here is an example:
```
gmt begin GMT_autolegend
gmt plot -R0/7.2/3/7.2 -Jx2c @Table_5_11.txt -Sc0.35c -Glightgreen -Wfaint -lApples+H"LEGEND"+f16p+D
gmt plot @Table_5_11.txt -St0.35c -Gorange -B -BWStr -lOranges
gmt legend -DjTR+w3c+o0.25c -F+p1p+ggray95+s+c10p/10p
gmt end show
```
Removing `+c10p/10p` and adding `-C10p/10p` gives similar plots.

For large clearances, `+c50p/50p` still works well, but `-C50p/50p` doesn't. Here is the result for `-C50p/50p`.