Remove row/column-spanning from docs source, fixes #4000

Sphinx doesn't support row or column-spanning, which caused the content
of the additional features table to be incorrect. To fix this, we now
define the content of each cell explicitly.

This change also fixes #3990.

docs/usage_general.rst.inc

@@ -435,26 +435,24 @@ On some platforms additional features are supported:
 
 .. Yes/No's are grouped by reason/mechanism/reference.
 
-+------------------+----------+-----------+------------+
-| Platform         | ACLs     | xattr     | Flags      |
-|                  | [#acls]_ | [#xattr]_ | [#flags]_  |
-+==================+==========+===========+============+
-| Linux            | Yes      | Yes       | Yes [1]_   |
-+------------------+----------+-----------+------------+
-| Mac OS X         | Yes      | Yes       | Yes (all)  |
-+------------------+----------+-----------+            |
-| FreeBSD          | Yes      | Yes       |            |
-+------------------+----------+-----------+            |
-| OpenBSD          | n/a      | n/a       |            |
-+------------------+----------+-----------+            |
-| NetBSD           | n/a      | No [2]_   |            |
-+------------------+----------+-----------+------------+
-| Solaris 11       | No [3]_              | n/a        |
-+------------------+                      |            |
-| OpenIndiana      |                      |            |
-+------------------+----------+-----------+------------+
-| Windows (cygwin) | No [4]_  | No        | No         |
-+------------------+----------+-----------+------------+
++-------------------------+----------+-----------+------------+
+| Platform                | ACLs     | xattr     | Flags      |
+|                         | [#acls]_ | [#xattr]_ | [#flags]_  |
++=========================+==========+===========+============+
+| Linux                   | Yes      | Yes       | Yes [1]_   |
++-------------------------+----------+-----------+------------+
+| Mac OS X                | Yes      | Yes       | Yes (all)  |
++-------------------------+----------+-----------+------------+
+| FreeBSD                 | Yes      | Yes       | Yes (all)  |
++-------------------------+----------+-----------+------------+
+| OpenBSD                 | n/a      | n/a       | Yes (all)  |
++-------------------------+----------+-----------+------------+
+| NetBSD                  | n/a      | No [2]_   | Yes (all)  |
++-------------------------+----------+-----------+------------+
+| Solaris and derivatives | No [3]_  | No [3]_   | n/a        |
++-------------------------+----------+-----------+------------+
+| Windows (cygwin)        | No [4]_  | No        | No         |
++-------------------------+----------+-----------+------------+
 
 Other Unix-like operating systems may work as well, but have not been tested at all.