trust revoke: docs for docker trust revoke

Signed-off-by: Riyaz Faizullabhoy <riyaz.faizullabhoy@docker.com>

docs/reference/commandline/trust_revoke.md

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+---
+title: "trust revoke"
+description: "The revoke command description and usage"
+keywords: "revoke, notary, trust"
+---
+
+<!-- This file is maintained within the docker/cli Github
+     repository at https://github.com/docker/cli/. Make all
+     pull requests against that repo. If you see this file in
+     another repository, consider it read-only there, as it will
+     periodically be overwritten by the definitive file. Pull
+     requests which include edits to this file in other repositories
+     will be rejected.
+-->
+
+# trust revoke
+
+```markdown
+Usage:  docker trust revoke [OPTIONS] IMAGE[:TAG]
+
+Remove trust for an image
+
+Options:
+      --help   Print usage
+  -y, --yes    Answer yes to the removal question (no confirmation)
+
+```
+
+## Description
+
+Docker trust revoke removes signatures from tags in signed repositories.
+
+## Examples
+
+### Revoke signatures from a signed tag
+
+Here's an example of a repo with two signed tags:
+
+
+```bash
+$ docker trust inspect example/trust-demo
+SIGNED TAG          DIGEST                                                              SIGNERS
+red                 852cc04935f930a857b630edc4ed6131e91b22073bcc216698842e44f64d2943    alice
+blue                f1c38dbaeeb473c36716f6494d803fbfbe9d8a76916f7c0093f227821e378197    alice, bob
+
+List of signers and their KeyIDs:
+
+SIGNER              KEYS
+alice               05e87edcaecb
+bob                 5600f5ab76a2
+
+Administrative keys for example/trust-demo:
+Repository Key:	ecc457614c9fc399da523a5f4e24fe306a0a6ee1cc79a10e4555b3c6ab02f71e
+Root Key:	3cb2228f6561e58f46dbc4cda4fcaff9d5ef22e865a94636f82450d1d2234949
+```
+
+When alice runs `docker trust revoke`  like so:
+
+```bash
+$ docker trust revoke example/trust-demo:red
+Enter passphrase for delegation key with ID 27d42a8:
+Successfully deleted signature for example/trust-demo:red
+```
+
+the said tag gets removed from the list of released tags:
+
+```bash
+$ docker trust inspect example/trust-demo
+SIGNED TAG          DIGEST                                                              SIGNERS
+blue                f1c38dbaeeb473c36716f6494d803fbfbe9d8a76916f7c0093f227821e378197    alice, bob
+
+List of signers and their KeyIDs:
+
+SIGNER              KEYS
+alice               05e87edcaecb
+bob                 5600f5ab76a2
+
+Administrative keys for example/trust-demo:
+Repository Key:	ecc457614c9fc399da523a5f4e24fe306a0a6ee1cc79a10e4555b3c6ab02f71e
+Root Key:	3cb2228f6561e58f46dbc4cda4fcaff9d5ef22e865a94636f82450d1d2234949
+```
+
+### Revoke signatures on all tags in a repository
+
+When no tag is specified, `docker trust` revokes all signatures that you have a signing key for.
+
+```bash
+$ docker trust inspect example/trust-demo
+SIGNED TAG          DIGEST                                                              SIGNERS
+red                 852cc04935f930a857b630edc4ed6131e91b22073bcc216698842e44f64d2943    alice
+blue                f1c38dbaeeb473c36716f6494d803fbfbe9d8a76916f7c0093f227821e378197    alice, bob
+
+List of signers and their KeyIDs:
+
+SIGNER              KEYS
+alice               05e87edcaecb
+bob                 5600f5ab76a2
+
+Administrative keys for example/trust-demo:
+Repository Key:	ecc457614c9fc399da523a5f4e24fe306a0a6ee1cc79a10e4555b3c6ab02f71e
+Root Key:	3cb2228f6561e58f46dbc4cda4fcaff9d5ef22e865a94636f82450d1d2234949
+```
+
+When alice runs `docker trust revoke`  like so:
+
+```bash
+$ docker trust revoke example/trust-demo
+Please confirm you would like to delete all signature data for example/trust-demo? (y/n) y
+Enter passphrase for delegation key with ID 27d42a8:
+Successfully deleted signature for example/trust-demo
+```
+
+the all tags that have alice's signature on them get removed from the list of released tags:
+
+```bash
+$ docker trust inspect example/trust-demo
+
+No signatures for example/trust-demo
+
+
+List of signers and their KeyIDs:
+
+SIGNER              KEYS
+alice               05e87edcaecb
+bob                 5600f5ab76a2
+
+Administrative keys for example/trust-demo:
+Repository Key:	ecc457614c9fc399da523a5f4e24fe306a0a6ee1cc79a10e4555b3c6ab02f71e
+Root Key:	3cb2228f6561e58f46dbc4cda4fcaff9d5ef22e865a94636f82450d1d2234949
+```
+