man: Clarify arguments to install scripts

APKBUILD.5

@@ -519,9 +519,9 @@ variable must contain the install scripts needed by the package.
 
 The install script will be run inside the root filesystem where the package is
 being installed.  A single argument will be passed to all scripts, which is the
-version of the package being installed.  The pre-upgrade and post-upgrade
-scripts will have an additional second argument, which is the old version that
-is currently installed on the system.
+version of the package being currently installed (or deinstalled).  The
+pre-upgrade and post-upgrade scripts will have an additional second argument,
+which specifies the version of the package before the upgrade process.
 
 The different actions that may have install scripts specified are as follows: