Clarify diff display of MutatingCommand

Tools inheriting from MutatingCommand print out the change they are going to
make and then ask the user to confirm that this is indeed what they wanted to
do.

The change is outputted as a list of updated values in the form

key -> [oldValue, newValue]

e.g.

dnsPaused -> [true, false]

This CL will change the output to be clearer:

key: oldValue -> newValue

e.g.

dnsPaused: true -> false

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170853745

docs/operational-procedures.md

@@ -63,7 +63,7 @@ Cursors can be updated as follows:
 $ nomulus -e {ENVIRONMENT} update_cursors exampletld --type RDE_STAGING \
     --timestamp 2016-09-01T00:00:00Z
 Update Cursor@ahFzfmRvbWFpbi1yZWdpc3RyeXIzCxIPRW50aXR5R3JvdXBSb290Igljcm9zcy10bGQMCxIIUmVnaXN0cnkiB3lvdXR1YmUM_RDE_STAGING
-cursorTime -> [2016-09-23T00:00:00.000Z, 2016-09-01T00:00:00.000Z]
+cursorTime: 2016-09-23T00:00:00.000Z -> 2016-09-01T00:00:00.000Z
 
 Perform this command? (y/N): Y
 Updated 1 entities.

docs/operational-procedures/premium-list-management.md

@@ -76,7 +76,7 @@ parameter:
 ```shell
 $ nomulus -e {ENVIRONMENT} update_tld exampletld --premium_list exampletld
 Update Registry@exampletld
-premiumList -> [null, Key<?>(EntityGroupRoot("cross-tld")/PremiumList("exampletld"))]
+premiumList: null -> Key<?>(EntityGroupRoot("cross-tld")/PremiumList("exampletld"))
 
 Perform this command? (y/N): y
 Updated 1 entities.

docs/operational-procedures/reserved-list-management.md

@@ -124,7 +124,7 @@ parameter:
 $ nomulus -e {ENVIRONMENT} update_tld exampletld \
     --add_reserved_lists common_bad-words
 Update Registry@exampletld
-reservedLists -> [null, [Key<?>(EntityGroupRoot("cross-tld")/ReservedList("common_bad-words"))]]
+reservedLists: null -> [Key<?>(EntityGroupRoot("cross-tld")/ReservedList("common_bad-words"))]
 Perform this command? (y/N): y
 Updated 1 entities.
 ```