Add /hash to spack help --spec (#10691)

* Add /hash to spack help --spec
* Make /hash gray, add ^/hash

lib/spack/spack/cmd/help.py

@@ -19,7 +19,9 @@
 
   package [constraints] [^dependency [constraints] ...]
 
-  package                           any package from 'spack list'
+  package                           any package from 'spack list', or
+  @K{/hash}                             unique prefix or full hash of
+                                    installed package
 
   constraints:
     versions:
@@ -51,6 +53,8 @@
 
     dependencies:
       ^dependency [constraints]     specify constraints on dependencies
+      ^@K{/hash}                        build with a specific installed
+                                    dependency
 
   examples:
       hdf5                          any hdf5 configuration