Charm: more info about build targets (#8713)

* Charm: more info about build targets

* fix whitespace

var/spack/repos/builtin/packages/charm/package.py

@@ -63,6 +63,7 @@ class Charm(Package):
     variant(
         "build-target",
         default="LIBS",
+        # AMPI also builds charm++, LIBS also builds AMPI and charm++
         values=("charm++", "AMPI", "LIBS"),
         description="Specify the target to build"
     )