This is something I had been asked for by an Arch newcomer.
The short version
$ pacman -Ql package-name
The long version
$ pacman --query --list package-name
What does the manual say?
-Q, --query Query the package database. This operation allows you to view installed packages and their files, as well as meta-information about individual packages (dependencies, conflicts, install date, build date, size). This can be run against the local package database or can be used on individual package files. In the first case, if no package names are provided in the command line, all installed packages will be queried. Additionally, various filters can be applied on the package list. See Query Options below.
-l, --list List all files owned by a given package. Multiple packages can be specified on the command line.