org-insert-subheading: Never insert sub-heading above current

* lisp/org.el (org-insert-subheading): When at bol, do not insert
heading above (default behavior of `org-insert-heading').  Instead,
force `org-insert-heading' to insert below.  Improve docstring,
explaining how the prefix arguments are used and the new different
with `org-insert-heading'.
* etc/ORG-NEWS (~org-insert-subheading~ no longer inserts a
sub-heading above current when point is at the beginning of line):
Document the breaking change.

Reported-by: Michael Dauer <mick.dauer@gmail.com>
Link: https://list.orgmode.org/CAP7OBx+whiB8Jo_hEcfL6MajDU1EH=E5j0ZOvHB3dPRK+Aj4mQ@mail.gmail.com/

etc/ORG-NEWS

@@ -13,6 +13,27 @@ Please send Org bug reports to mailto:emacs-orgmode@gnu.org.
 
 * Version 9.7 (not released yet)
 ** Important announcements and breaking changes
+*** ~org-insert-subheading~ no longer inserts a sub-heading above current when point is at the beginning of line
+
+Previously, calling ~org-insert-subheading~ on
+
+: * Heading 1
+: <point>* Heading 2
+
+yielded
+
+: * Heading 1
+: ** <point>
+: * Heading 2
+
+This is no longer the case.  The sub-heading is always created below
+current heading (prefix arguments have the same meaning as in
+~org-insert-heading~):
+
+: * Heading 1
+: * Heading 2
+: ** <point>
+
 *** Org mode now fontifies whole table lines (including newline) according to ~org-table~ face
 
 Previously, leading indentation and trailing newline in table rows

lisp/org.el

@@ -6633,8 +6633,12 @@ unchecked check box."
 
 (defun org-insert-subheading (arg)
   "Insert a new subheading and demote it.
-Works for outline headings and for plain lists alike."
+Works for outline headings and for plain lists alike.
+The prefix argument ARG is passed to `org-insert-heading'.
+Unlike `org-insert-heading', when point is at the beginning of a
+heading, still insert the new sub-heading below."
   (interactive "P")
+  (when (bolp) (forward-char))
   (org-insert-heading arg)
   (cond
    ((org-at-heading-p) (org-do-demote))