doc: Move virtualfile_from_* methods as low-level API functions (#3096)

Author: seisman

doc/api/index.rst

@@ -289,9 +289,6 @@ Python objects to and from GMT virtual files:
 .. autosummary::
     :toctree: generated
 
-    clib.Session.virtualfile_from_matrix
-    clib.Session.virtualfile_from_vectors
-    clib.Session.virtualfile_from_grid
     clib.Session.virtualfile_in
     clib.Session.virtualfile_out
     clib.Session.virtualfile_to_dataset
@@ -317,3 +314,7 @@ Low level access (these are mostly used by the :mod:`pygmt.clib` package):
     clib.Session.read_virtualfile
     clib.Session.extract_region
     clib.Session.get_libgmt_func
+    clib.Session.virtualfile_from_data
+    clib.Session.virtualfile_from_grid
+    clib.Session.virtualfile_from_matrix
+    clib.Session.virtualfile_from_vectors