Figure.grdcontour: Adjust processing of arguments passed to the "annotation" and "interval" parameters, deprecate "sequence_plus"

pygmt/src/grdcontour.py

@@ -27,9 +27,7 @@
     p="perspective",
     t="transparency",
 )
-@kwargs_to_strings(
-    R="sequence", L="sequence", A="sequence_plus", c="sequence_comma", p="sequence"
-)
+@kwargs_to_strings(R="sequence", L="sequence", c="sequence_comma", p="sequence")
 def grdcontour(self, grid, **kwargs):
     r"""
     Convert grids or images to contours and plot them on maps.
@@ -53,16 +51,17 @@ def grdcontour(self, grid, **kwargs):
           angle (col 3).
         - A fixed contour interval *cont_int* or a single contour with
           +\ *cont_int*.
-    annotation : str, int, or list
+    annotation : float or str
         Specify or disable annotated contour levels, modifies annotated
         contours specified in ``interval``.
 
         - Specify a fixed annotation interval *annot_int* or a
-          single annotation level +\ *annot_int*.
-        - Disable all annotation with  **-**.
-        - Optional label modifiers can be specified as a single string
-          ``"[annot_int]+e"``  or with a list of arguments
-          ``([annot_int], "e", "f10p", "gred")``.
+          single annotation level **+**\ *annot_int*.
+        - Disable all annotations by appending **n**.
+        - The appearance can be adjusted by appending different modifiers, e.g.,
+          ``"annot_int+f10p+gred"`` gives annotations with a font size of 10
+          points and a red filled box. For all available modifiers see
+          :gmt-docs:`grdcontour.html#a`.
     limit : str or list of 2 ints
         *low*/*high*.
         Do no draw contours below `low` or above `high`, specify as string