mrtest.apk_installed: Add docstring for get_installed_all

mrtest/apk_installed.py

@@ -11,6 +11,8 @@ cache_installed_all = None
 
 
 def get_installed_all() -> dict[str, Optional[str]]:
+    """Returns a map of all installed packages containing their names mapped to their
+    versions. The version is None only if the package is a virtual package."""
     global cache_installed_all
 
     if cache_installed_all: