DOCS/man/input: note that properties can be unavailable on init

It comes up often in IRC and issues that users don't understand why the
path property is initially unavailable, so link the section that
mentions it from the Properties section, and expand on how to get these
properties.

DOCS/man/input.rst

@@ -1940,6 +1940,9 @@ If an option is referenced, the property will normally take/return exactly the
 same values as the option. In these cases, properties are merely a way to change
 an option at runtime.
 
+Note that many properties are unavailable at startup. See `Details on the script
+initialization and lifecycle`_.
+
 Property list
 -------------
 

DOCS/man/lua.rst

@@ -86,7 +86,8 @@ own event handlers which you have registered with ``mp.register_event``, or
 timers added with ``mp.add_timeout`` or similar. Note that since the
 script starts execution concurrently with player initialization, some properties
 may not be populated with meaningful values until the relevant subsystems have
-initialized.
+initialized. Rather than retrieving these properties at the top of scripts, you
+should use ``mp.observe_property`` or read them within event handlers.
 
 When the player quits, all scripts will be asked to terminate. This happens via
 a ``shutdown`` event, which by default will make the event loop return. If your