Minor docs improvements (#386)

Update B decorator to "str or list" and add alias `map_scale` for L to `basemap`.

pygmt/base_plotting.py

@@ -64,6 +64,7 @@ def _preprocess(self, **kwargs):  # pylint: disable=no-self-use
         B="frame",
         D="resolution",
         I="rivers",
+        L="map_scale",
         N="borders",
         W="shorelines",
         G="land",
@@ -115,6 +116,9 @@ def coast(self, **kwargs):
             ``'river[/pen]'``
             Draw rivers. Specify the type of rivers and [optionally] append pen
             attributes.
+        L : str
+            ``'[g|j|J|n|x]refpoint'``
+            Draws a simple map scale centered on the reference point specified.
         N : str
             ``'border[/pen]'``
             Draw political boundaries. Specify the type of boundary and

pygmt/helpers/decorators.py

@@ -23,7 +23,7 @@
             *Required if this is the first plot command*.
             Select map projection.""",
     "B": """\
-        B : str
+        B : str or list
             Set map boundary frame and axes attributes.""",
     "U": """\
         U : bool or str