ref/cmd/principal: Add more blessing information getters.

This closes veyron/release-issues#991.

This change add the following commands to make driling into
blessings more command line friendly:
(1) 'principal get publickey' gets the publickey of the principal.
(2) 'principal get default' and 'principal get forpeer <peer>'
    now take the following flags.
    (a) 'principal get default --names' prints the names of the blessings.
    (b) 'principal get default --rootkey <chainname> prints the root key of
        one of the names from (a).
    (c) 'principal get default --caveats <chainname> prints the caveats of
        one fo the names from (a).

Change-Id: I00754e2dd047748931c9be504033118e3171ca68
diff --git a/cmd/principal/doc.go b/cmd/principal/doc.go
index 93a5977..9ffcd6d 100644
--- a/cmd/principal/doc.go
+++ b/cmd/principal/doc.go
@@ -223,7 +223,7 @@
 
 The principal dump flags are:
  -s=false
-   If true, show a only the default blessing names
+   If true, show only the default blessing names
 
 Principal Dumpblessings
 
@@ -389,6 +389,7 @@
 The principal get commands are:
    default         Return blessings marked as default
    forpeer         Return blessings marked for the provided peer
+   publickey       Prints the public key of the principal.
    recognizedroots Return recognized blessings, and their associated public key.
    peermap         Shows the map from peer pattern to which blessing name to
                    present.
@@ -396,22 +397,29 @@
 Principal Get Default
 
 Returns blessings that are marked as default in the BlessingStore specified by
-the environment that this tool is running in.
+the environment that this tool is running in. Providing --names will print the
+default blessings' chain names. Providing --rootkey <chain_name> will print the
+root key of the certificate chain with chain_name. Providing --caveats
+<chain_name> will print the caveats on the certificate chain with chain_name.
 
 Usage:
    principal get default [flags]
 
 The principal get default flags are:
- -name=false
+ -caveats=
+   Shows the caveats on the provided certificate chain name.
+ -names=false
    If true, shows the value of the blessing name to be presented to the peer
- -rootkey=false
-   If true, shows the value of the root key of the certificate chain to be
-   presented to the peer
+ -rootkey=
+   Shows the value of the root key of the provided certificate chain name.
 
 Principal Get Forpeer
 
 Returns blessings that are marked for the provided peer in the BlessingStore
-specified by the environment that this tool is running in.
+specified by the environment that this tool is running in. Providing --names
+will print the blessings' chain names. Providing --rootkey <chain_name> will
+print the root key of the certificate chain with chain_name. Providing --caveats
+<chain_name> will print the caveats on the certificate chain with chain_name.
 
 Usage:
    principal get forpeer [flags] [<peer_1> ... <peer_k>]
@@ -423,11 +431,20 @@
 the "..." pattern).
 
 The principal get forpeer flags are:
- -name=false
+ -caveats=
+   Shows the caveats on the provided certificate chain name.
+ -names=false
    If true, shows the value of the blessing name to be presented to the peer
- -rootkey=false
-   If true, shows the value of the root key of the certificate chain to be
-   presented to the peer
+ -rootkey=
+   Shows the value of the root key of the provided certificate chain name.
+
+Principal Get Publickey
+
+Prints out the public key of the principal specified by the environment that
+this tool is running in.
+
+Usage:
+   principal get publickey
 
 Principal Get Recognizedroots