[20.10 backport] cli: additionalHelp() don't decorate output if it's piped, and add extra newline

[thaJeztah]

• backport of cli: additionalHelp() don't decorate output if it's piped, and add extra newline #3973

Relates to:

• Help guides link #2857
• Help link: remove color, add "bold" style, and white-space #2884
• 404 at /go/guides/%1B[0m docs#12488
• 404 at /go/guides/%1B[0m docs#13146
• 404 at /go/guides/github.com/dockersamples/example-voting-app docs#13838
• 404 at /go%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20/guides/ docs#14008
• 404 at /go/guides/%1B[0m docs#15583
• 404 at /go/guides/%1B[0m docs#15584
• 404 at /go/guides/PS docs#15599
• 404 at /go/guides/ESC docs#16464

cli: additionalHelp() don't decorate output if it's piped

This prevents the escape-characters being included when piping the
output, e.g. docker --help > output.txt, or docker --help | something.
These control-characters could cause issues if users copy/pasted the URL
from the output, resulting in them becoming part of the URL they tried
to visit, which would fail, e.g. when copying the output from:

To get more help with docker, check out our guides at https://docs.docker.com/go/guides/

Users ended up on URLs like;

https://docs.docker.com/go/guides/ESC
https://docs.docker.com/go/guides/%1B[0m

Before this patch, control characters ("bold") would be printed, even if
no TTY was attached;

docker --help > output.txt
cat output.txt | grep 'For more help' | od -c
0000000 033   [   1   m   F   o   r       m   o   r   e       h   e   l
0000020   p       o   n       h   o   w       t   o       u   s   e
0000040   D   o   c   k   e   r   ,       h   e   a   d       t   o
0000060   h   t   t   p   s   :   /   /   d   o   c   s   .   d   o   c
0000100   k   e   r   .   c   o   m   /   g   o   /   g   u   i   d   e
0000120   s   / 033   [   0   m  \n
0000127

docker --help | grep 'For more help' | od -c
0000000 033   [   1   m   F   o   r       m   o   r   e       h   e   l
0000020   p       o   n       h   o   w       t   o       u   s   e
0000040   D   o   c   k   e   r   ,       h   e   a   d       t   o
0000060   h   t   t   p   s   :   /   /   d   o   c   s   .   d   o   c
0000100   k   e   r   .   c   o   m   /   g   o   /   g   u   i   d   e
0000120   s   / 033   [   0   m  \n
0000127

With this patch, no control characters are included:

docker --help > output.txt
cat output.txt | grep 'For more help' | od -c
0000000   F   o   r       m   o   r   e       h   e   l   p       o   n
0000020       h   o   w       t   o       u   s   e       D   o   c   k
0000040   e   r   ,       h   e   a   d       t   o       h   t   t   p
0000060   s   :   /   /   d   o   c   s   .   d   o   c   k   e   r   .
0000100   c   o   m   /   g   o   /   g   u   i   d   e   s   /  \n
0000117

docker --help | grep 'For more help' | od -c
0000000   F   o   r       m   o   r   e       h   e   l   p       o   n
0000020       h   o   w       t   o       u   s   e       D   o   c   k
0000040   e   r   ,       h   e   a   d       t   o       h   t   t   p
0000060   s   :   /   /   d   o   c   s   .   d   o   c   k   e   r   .
0000100   c   o   m   /   g   o   /   g   u   i   d   e   s   /  \n
0000117

Add extra newline after additionalHelp output

The additionalHelp message is printed at the end of the --help output;

To get more help with docker, check out our guides at https://docs.docker.com/go/guides/
PS>

As this message may contain an URL, users may copy/paste the URL to open it
in their browser, but can easily end up copying their prompt (as there's
no whitespace after it), and as a result end up on a broken URL, for example:

https://docs.docker.com/go/guides/PS

This patch adds an extra newline at the end to provide some whitespace
around the message, making it less error-prone to copy the URL;

To get more help with docker, check out our guides at https://docs.docker.com/go/guides/

PS>

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Thx! I think this one should be fairly save to include, and hopefully reduces some of the "404" reports in our docs, so let me get this one in.