Clarify interplay of spacing and per_column in info (#1127)

Author: core-man

pygmt/src/info.py

@@ -54,9 +54,11 @@ def info(table, **kwargs):
         Report the min/max values per column in separate columns.
     spacing : str
         [**b**\|\ **p**\|\ **f**\|\ **s**]\ *dx*\[/*dy*\[/*dz*...]].
-        Report the min/max of the first n columns to the nearest multiple of
-        the provided increments and output results in the form
-        ``[w, e, s, n]``.
+        Compute the min/max values of the first n columns to the nearest
+        multiple of the provided increments [default is 2 columns]. By default,
+        output results in the form ``[w, e, s, n]``, unless ``per_column`` is
+        set in which case we output each min and max value in separate output
+        columns.
     nearest_multiple : str
         **dz**\[\ **+c**\ *col*].
         Report the min/max of the first (0'th) column to the nearest multiple