Update explain_-B.rst_ (#7667)

Proof of undefined results:
```
gmt basemap -R-10/10/-10/10     -JM5i -Bagf -B+o-11/-11 -png OK1
gmt basemap -R-10/10/-10/10 -fg -JX5i -Bagf -B+o-11/-11 -png OK2
gmt basemap -R-10/10/-10/10     -JX5i -Bagf -B+o-11/-11 -png BAD1
gmt basemap -R10/20/10/20       -JX5i -Bagf -B+o-11/-11 -png BAD2

```

doc/rst/source/explain_-B.rst_

@@ -71,7 +71,8 @@ can be appended to **-B** to control the Frame settings:
   **0**].
 
 - **+o**\ *lon/lat* to produce oblique gridlines about another pole specified by *lon/lat* [default references to the
-  North pole]. **+o** is ignored if no gridlines are requested.
+  North pole]. **+o** is ignored if no gridlines are requested. Note, one cannot specify oblique gridlines for the
+  oblique Mercator projection, and results are not currently defined for non-geographic projections.
 
 - **+n** to have no frame and annotations at all [default is controlled by *axes*].