New comment by Chocimier on void-packages repository

https://github.com/void-linux/void-packages/pull/32560#issuecomment-904009030

Comment:
Added `-f`, simplified `XBPS_TARGET_MACHINE`.

As for unsetting XBPS_PRESERVE_PKGS with flag, is this really more useful than unsetting XBPS_CHECK_PKGS? Do we want parity  between flags and variables in general?

For documentation, anything more needed?

> if there suddenly appears a new subpackage

Well, as proposed, xbps-src only looks whether package from command line argument is there, and if not, it overrides all other packages build together. IMO good enough and better than as is, i.e. preserve existing packages but index new, possibly with conflicting files.