Document the -R+U option in the global -R

Fix #7537

doc/rst/source/explain_-R.rst_

@@ -5,7 +5,7 @@ The **-R** option
 
 .. _-R:
 
-**-R**\ *xmin*/*xmax*/*ymin*/*ymax*\ [**+r**][**+u**\ *unit*]
+**-R**\ *xmin*/*xmax*/*ymin*/*ymax*\ [**+r**][**+u|U**\ *unit*]
     Specify the region of interest. |Add_-R|
 
 **Description**
@@ -17,12 +17,12 @@ The **-R** option defines the map region or data domain of interest. It may be s
    geographic regions when using map projections where meridians and parallels are rectilinear, where *xmin*, *xmax*,
    *ymin*, and *ymax* refer to the data limits.
 
-#. **-R**\ *xmin*/*xmax*/*ymin*/*ymax*\ **+u**\ *unit*. Append **+u**\ *unit* to the option 1 to specify a region in
+#. **-R**\ *xmin*/*xmax*/*ymin*/*ymax*\ **+u|U**\ *unit*. Append **+u**\ *unit* to the option 1 to specify a region in
    projected units (e.g., UTM meters) where *xmin*/*xmax*/*ymin*/*ymax* are Cartesian projected coordinates compatible
    with the chosen projection and *unit* is an allowable :ref:`distance unit <dist-units>` [e]. The coordinates are relative
-   to the standard longitude and latitude indicated in the projection (**-J**).  For projected regions centered on
+   to the standard longitude and latitude indicated in the projection (**-J**). For projected regions centered on
    (0,0) you may use the short-hand **-R**\ *halfwidth*\ [/*halfheight*]\ **+u**\ *unit*, where *halfheight* defaults
-   to *halfwidth* if not given. This short-hand requires the **+u** modifier.
+   to *halfwidth* if not given. This short-hand requires the **+u** modifier. For convenience, we also support the inverse translation, i.e., +Uunit. This modifier can be used to convert your grid coordinates from meters to the specified unit.
 
 #. **-R**\ *xlleft*/*ylleft*/*xuright*/*yuright*\ **+r**. This form is useful for map projections that are oblique,
    making meridians and parallels poor choices for map boundaries. Here, we instead specify the lower left corner and