man: Better document install scripts

APKBUILD.5

@@ -515,8 +515,15 @@ An install script must be written in shell and must have a
 .Li Dq #!/bin/sh
 interpreter declaration as the first line.  The
 .Cm install
-variable must contain the install scripts needed by the package.  The different
-actions available are as follows:
+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.
+
+The different actions that may have install scripts specified are as follows:
 
 .Bl -tag -width Ds
 .It Ic $pkgname.pre-install