Huh, but it does say in VCS package guidelines?
If the resulting package is different after changing e.g. the dependencies, URL or sources — update pkgver to the latest version.
It is recommended to have following version format: RELEASE.rREVISION
And regarding the function:
The pkgver autobump is now achieved via a dedicated pkgver() function. This allows for better control over the pkgver, and maintainers should favor a pkgver that makes sense. To use pkgver(), you still need to declare the pkgver variable with the most recent value. makepkg will invoke function pkgver(), and update variable pkgver accordingly.
So yeah, you just put the output of pkgver() into the pkgver variable initially, and then each repo update automatically updates the pkgver variable, as far as I understand it.