Add alias for blockmean's -S parameter

pygmt/src/blockm.py

@@ -70,6 +70,7 @@ def _blockm(block_method, data, x, y, z, outfile, **kwargs):
 @use_alias(
     I="spacing",
     R="region",
+    S="restype",
     V="verbose",
     a="aspatial",
     b="binary",
@@ -111,6 +112,12 @@ def blockmean(data=None, x=None, y=None, z=None, outfile=None, **kwargs):
 
     {I}
 
+    restype: str
+        Result type calculated by blockmean. Use **n** to report the number of
+        input points inside each block, **s** to report the sum of all z-values
+        inside a block, **w** to report the sum of weights [Default or **m**
+        reports mean value].
+
     {R}
 
     outfile : str