source_cache, misc_cache, verify_ssl, checksum, & dirty in config.yaml

- Added new preferences to config.yaml:
  - source_cache
  - misc_cache
  - verify_ssl
  - checksum
  - dirty

etc/spack/defaults/config.yaml

@@ -35,6 +35,8 @@ config:
   # A value of $spack/var/spack/stage indicates that Spack should run
   # builds directly inside its install directory without staging them in
   # temporary space.
+  #
+  # The build stage can be purged with `spack purge --stage`.
   build_stage:
     - $tempdir
     - /nfs/tmp2/$user
@@ -42,11 +44,12 @@ config:
 
 
   # Cache directory already downloaded source tarballs and archived
-  # repositories. This can be purged with spack purge
+  # repositories. This can be purged with `spack purge --downloads`.
   source_cache: $spack/var/spack/cache
 
 
   # Cache directory for miscellaneous files, like the package index.
+  # This can be purged with `spack purge --misc-cache`
   misc_cache: ~/.spack/cache