Improve the documentation of Figure.shift_origin() (#536)

- Fix a broken link
- Add a note that the function can't be used as the fisrt plotting command.

Author: seisman

pygmt/figure.py

@@ -298,7 +298,7 @@ def shift_origin(self, xshift=None, yshift=None):
         Shift plot origin in x and/or y directions.
 
         This method shifts plot origin relative to the current origin by
-        (*xshift*,*yshift*) and optionally append the length unit (**c**,
+        (*xshift*, *yshift*) and optionally append the length unit (**c**,
         **i**, or **p**).
 
         Prepend **a** to shift the origin back to the original position after
@@ -308,14 +308,22 @@ def shift_origin(self, xshift=None, yshift=None):
         move the origin relative to its current location.
 
         Detailed usage at
-        :gmt-docs:`GMT_Docs.html#plot-positioning-and-layout-the-x-y-options`
+        :gmt-docs:`cookbook/options.html#plot-positioning-and-layout-the-x-y-options`
 
         Parameters
         ----------
         xshift : str
             Shift plot origin in x direction.
         yshift : str
             Shift plot origin in y direction.
+
+        Notes
+        -----
+        For GMT 6.1.0, this function can't be used as the first plotting
+        function of :meth:`pygmt.Figure`, since it relies the *region* and
+        *projection* settings from previous commands.
+
+        .. TODO: Remove the notes when PyGMT bumps to GMT>=6.1.1.
         """
         self._preprocess()
         args = ["-T"]