Updating doc string for cyl_mercator.py

examples/projections/cyl/cyl_mercator.py

@@ -12,8 +12,11 @@
 extensively for world maps in which the distortion towards the polar regions grows
 rather large.
 
-``M[lon0/][lat0/]width``: Give central meridian ``lon0`` (optional) and
-standard parallel ``lat0`` (optional).
+**m**\ [*lon0[/lat0]*]\ */scale* or **M**\ [*lon0*][*/lat0*]\ */width*
+
+The projection is set with **m** or **M**. The central meridian is set with the
+option *lon0* and the standard parallel is set with the option *lat0*.
+The figure size is set with *scale* or *width*.
 """
 import pygmt