PKGBUILD.5: document that the pkgver() function runs after prepare()

Signed-off-by: Allan McRae <allan@archlinux.org>

doc/PKGBUILD.5.txt

@@ -52,10 +52,10 @@ similar to `$_basekernver`.
 +
 The `pkgver` variable can be automatically updated by providing a `pkgver()`
 function in the PKGBUILD that outputs the new package version.
-This is run after downloading and extracting the sources so it can use those
-files in determining the new `pkgver`.
-This is most useful when used with sources from version control systems (see
-below).
+This is run after downloading and extracting the sources and running the
+`prepare()` function (if present), so it can use those files in determining the
+new `pkgver`.  This is most useful when used with sources from version control
+systems (see below).
 
 *pkgrel*::
 	This is the release number specific to the Arch Linux release. This