vo_direct3d: fix texture-memory sub-option, extend it

This sub-option was turned into a flag when the sub-option parser was
changed to the generic one (probably accidentally). Turn it into a
proper choice-option.

Also, adjust what the options do. Though none of this probably makes
much sense; the default should work, and if it doesn't, the GPU/driver
is probably beyond help.

DOCS/man/vo.rst

@@ -233,18 +233,29 @@ Available video output drivers are:
         Always force textures to power of 2, even if the device reports
         non-power-of-2 texture sizes as supported.
 
-    ``texture-memory=N``
+    ``texture-memory=<mode>``
         Only affects operation with shaders/texturing enabled, and (E)OSD.
-        Values for N:
+        Possible values:
 
-            0
-                default, will often use an additional shadow texture + copy
-            1
-                use ``D3DPOOL_MANAGED``
-            2
-                use ``D3DPOOL_DEFAULT``
-            3
-                use ``D3DPOOL_SYSTEMMEM``, but without shadow texture
+        ``default`` (default)
+            Use ``D3DPOOL_DEFAULT``, with a ``D3DPOOL_SYSTEMMEM`` texture for
+            locking. If the driver supports ``D3DDEVCAPS_TEXTURESYSTEMMEMORY``,
+            ``D3DPOOL_SYSTEMMEM`` is used directly.
+
+        ``default-pool``
+            Use ``D3DPOOL_DEFAULT``. (Like ``default``, but never use a
+            shadow-texture.)
+
+        ``default-pool-shadow``
+            Use ``D3DPOOL_DEFAULT``, with a ``D3DPOOL_SYSTEMMEM`` texture for
+            locking. (Like ``default``, but always force the shadow-texture.)
+
+        ``managed``
+            Use ``D3DPOOL_MANAGED``.
+
+        ``scratch``
+            Use ``D3DPOOL_SCRATCH``, with a ``D3DPOOL_SYSTEMMEM`` texture for
+            locking.
 
     ``swap-discard``
         Use ``D3DSWAPEFFECT_DISCARD``, which might be faster.