command: mark get_property as deprecated

Using it just makes no sense. But we're really being nice about this and
don't remove it immediately.

(cherry picked from commit fd5403cb40)

DOCS/man/input.rst

@@ -618,9 +618,8 @@ Input Commands that are Possibly Subject to Change
     unseekable streams that are going out of sync.
     This command might be changed or removed in the future.
 
-Undocumented commands: ``tv_last_channel`` (TV/DVB only), ``get_property`` (?),
-``ao_reload`` (experimental/internal).
-
+Undocumented commands: ``tv_last_channel`` (TV/DVB only),
+``get_property`` (deprecated), ``ao_reload`` (experimental/internal).
 
 Hooks
 ~~~~~

player/command.c

@@ -4329,6 +4329,10 @@ int run_command(MPContext *mpctx, mp_cmd_t *cmd)
         }
         MP_INFO(mpctx, "ANS_%s=%s\n", cmd->args[0].v.s, tmp);
         talloc_free(tmp);
+        MP_WARN(mpctx,  "The get_property command is deprecated and "
+                        "will be removed in the next release.\n"
+                        "Use libmpv or the JSON IPC. "
+                        "(Or print_text, if you must.)");
         break;
     }