properly documented the seedwatch plugin

Because telling users to try the help function *isn't actually
documentation*.  Info pulled from source code without verification.

Readme.rst

@@ -1626,10 +1626,31 @@ Fortress activity management
 
 seedwatch
 ---------
-Tool for turning cooking of seeds and plants on/off depending on how much you
-have of them.
+Watches the numbers of seeds available and enables/disables seed and plant cooking.
 
-See 'seedwatch help' for detailed description.
+Each plant type can be assigned a limit. If their number falls below that limit,
+the plants and seeds of that type will be excluded from cookery.
+If the number rises above the limit + 20, then cooking will be allowed.
+
+The plugin needs a fortress to be loaded and will deactivate automatically otherwise.
+You have to reactivate with 'seedwatch start' after you load the game.
+
+Options:
+
+:all:       Adds all plants from the abbreviation list to the watch list.
+:start:     Start watching.
+:stop:      Stop watching.
+:info:      Display whether seedwatch is watching, and the watch list.
+:clear:     Clears the watch list.
+
+Examples:
+
+``seedwatch MUSHROOM_HELMET_PLUMP 30``
+    add ``MUSHROOM_HELMET_PLUMP`` to the watch list, limit = 30
+``seedwatch MUSHROOM_HELMET_PLUMP``
+    removes ``MUSHROOM_HELMET_PLUMP`` from the watch list.
+``seedwatch all 30``
+    adds all plants from the abbreviation list to the watch list, the limit being 30.
 
 zone
 ----