Add docs to settingtypes.txt

builtin/settingtypes.txt

@@ -57,6 +57,25 @@
 # Comments and (Readable name) are handled by gettext.
 # Comments should be complete sentences that describe the setting and possibly
 #  give the user additional useful insight.
+# The last line of a comment can contain the requirements for the setting, ie:
+#
+#    # This is a comment
+#    #
+#    # Requires: shaders, enable_dynamic_shadows, !touchscreen_gui
+#    name (Readable name) type type_args
+#
+# A requirement can be the name of a boolean setting or an engine-defined value.
+# These requirements may be:
+#
+# * The value of a boolean setting, such as enable_dynamic_shadows
+# * An engine-defined value:
+#     * shaders_support (a video driver that supports shaders, may not be enabled)
+#     * shaders (both enable_shaders and shaders_support)
+#     * desktop / android
+#     * touchscreen_gui / keyboard_mouse
+#     * opengl / gles
+# * You can negate any requirement by prepending with !
+#
 # Sections are marked by a single line in the format: [Section Name]
 # Sub-section are marked by adding * in front of the section name: [*Sub-section]
 # Sub-sub-sections have two * etc.