(Fbeginning_of_line, Fend_of_line): Clarify confusing doc string.

src/cmds.c

@@ -157,13 +157,13 @@ DEFUN ("beginning-of-line", Fbeginning_of_line, Sbeginning_of_line, 0, 1, "p",
 With argument N not nil or 1, move forward N - 1 lines first.
 If point reaches the beginning or end of buffer, it stops there.
 
-This command does not move point across a field boundary unless doing so
-would move beyond there to a different line; If N is nil or 1, and point
-starts at a field boundary, point does not move.  To ignore field
-boundaries, either bind `inhibit-field-text-motion' to t, or use the
-`forward-line' function instead.  For instance, `(forward-line 0)' does
-the same thing as `(beginning-of-line)', except that it ignores field
-boundaries.  */)
+This function does not move point across a field boundary unless that
+would move point to a different line than the original, unconstrained
+result.  If N is nil or 1, and a front-sticky field starts at point,
+the point does not move.  To ignore field boundaries bind
+`inhibit-field-text-motion' to t, or use the `forward-line' function
+instead.  For instance, `(forward-line 0)' does the same thing as
+`(beginning-of-line)', except that it ignores field boundaries.  */)
      (n)
      Lisp_Object n;
 {
@@ -183,10 +183,11 @@ With argument N not nil or 1, move forward N - 1 lines first.
 If point reaches the beginning or end of buffer, it stops there.
 To ignore intangibility, bind `inhibit-point-motion-hooks' to t.
 
-This command does not move point across a field boundary unless doing so
-would move beyond there to a different line; if N is nil or 1, and
-point starts at a field boundary, point does not move.  To ignore field
-boundaries bind `inhibit-field-text-motion' to t.  */)
+This function does not move point across a field boundary unless that
+would move point to a different line than the original, unconstrained
+result.  If N is nil or 1, and a rear-sticky field ends at point,
+the point does not move.  To ignore field boundaries bind
+`inhibit-field-text-motion' to t.  */)
      (n)
      Lisp_Object n;
 {