Figure.shift_origin: Improve docstrings to clarify the meaning of w/h (#4247)

Co-authored-by: Yvonne Fröhlich <94163266+yvonnefroehlich@users.noreply.github.com>

Author: seisman

pygmt/src/shift_origin.py

@@ -38,23 +38,23 @@ def shift_origin(
     to other units via :gmt-term:`PROJ_LENGTH_UNIT`. Optionally, append the length unit
     (**c** for centimeters, **i** for inches, or **p** for points) to the shifts.
 
-    For *xshift*, a special character **w** can also be used, which represents the
-    bounding box **width** of the previous plot. The full syntax is
-    [[±][*f*]\ **w**\ [/\ *d*\ ]±]\ *xoff*, where optional signs, factor *f* and divisor
-    *d* can be used to compute an offset that may be adjusted further by ±\ *xoff*.
-    Assuming that the previous plot has a width of 10 centimeters, here are some example
+    For *xshift*, character **w** can be used, which represents the bounding box
+    **width** of the last plotting object. The full syntax is
+    [[±][*f*]\ **w**\ [/*d*]±]\ *xoff*, where optional signs, factor *f* and divisor *d*
+    can be used to compute an offset that may be adjusted further by ±\ *xoff*. Assuming
+    that the last plotting object has a width of 10 centimeters, here are some example
     values for *xshift*:
 
     - ``"w"``: x-shift is 10 cm
     - ``"w+2c"``: x-shift is 10+2=12 cm
     - ``"2w+3c"``: x-shift is 2*10+3=23 cm
     - ``"w/2-2c"``: x-shift is 10/2-2=3 cm
 
-    Similarly, for *yshift*, a special character **h** can also be used, which is the
-    bounding box **height** of the previous plot.
+    Similarly, for *yshift*, character **h** can be used, which is the bounding box
+    **height** of the last plotting object.
 
-    **Note**: The previous plot bounding box refers to the last object plotted, which
-    may be a basemap, image, logo, legend, colorbar, etc.
+    **Note**: The last plotting object can be a basemap, image, logo, legend, colorbar,
+    etc.
 
     Parameters
     ----------