Deprecate -f flag from docker tag

Closes #9798

@maintainers please note that this is a change to the UX. We no longer
require the -f flag on `docker tag` to move a tag from an existing image.
However, this does make us more consistent across our commands,
see https://github.com/docker/docker/issues/9798 for the history.

Signed-off-by: Doug Davis <dug@us.ibm.com>

docs/reference/commandline/tag.md

@@ -14,7 +14,6 @@ parent = "smn_cli"
 
     Tag an image into a repository
 
-      -f, --force=false    Force the tagging even if there's a conflict
       --help=false         Print usage
 
 You can group your images together using names and tags, and then upload them

man/docker-tag.1.md

@@ -6,7 +6,6 @@ docker-tag - Tag an image into a repository
 
 # SYNOPSIS
 **docker tag**
-[**-f**|**--force**[=*false*]]
 [**--help**]
 IMAGE[:TAG] [REGISTRY_HOST/][USERNAME/]NAME[:TAG]
 
@@ -18,9 +17,6 @@ If you do not specify a `REGISTRY_HOST`, the command uses Docker's public
 registry located at `registry-1.docker.io` by default. 
 
 # "OPTIONS"
-**-f**, **--force**=*true*|*false*
-   When set to true, force the alias. The default is *false*.
-
 **--help**
    Print usage statement.