docs: document -aliasdef

docs/qbsp.rst

@@ -197,6 +197,55 @@ Options
    Set number of threads to use. By default, qbsp will attempt to
    use all available hardware threads.
 
+.. option:: -aliasdef <aliases.def> [...]
+
+   Adds alias definition files, which can transform entities in the .map into other entities.
+
+   For example, given this alias definition file:
+
+   .. code-block:: none
+      :caption: aliases.def
+
+      misc_torch1 // source classname
+      {
+      "classname" "misc_model" // classname to transform into
+      "model" "torch1.mdl"
+      }
+
+      misc_torch2
+      {
+      "classname" "misc_model"
+      "model" "torch2.mdl"
+      }
+
+   and an input map file:
+
+   .. code-block:: none
+
+      {
+      "classname" "misc_torch1"
+      "model" "override.mdl"
+      }
+
+      {
+      "classname" "misc_torch2"
+      }
+
+   the following will be output in the .bsp's entity lump:
+
+   .. code-block:: none
+
+      {
+      "classname" "misc_model"
+      "model" "override.mdl" // key/value from map takes precedence
+      }
+
+      {
+      "classname" "misc_model"
+      "model" "torch2.mdl" // key/value from alias file
+      }
+
+
 Game Path Specification
 -----------------------