Document managing sensitive strings (secrets)

[mdlinville]

Problem description

Document feature described in:

• Secret Management moby/moby#27794
• only check secrets for service create if requested moby/moby#28306

Project version(s) affected

Engine 1.13

Suggestions for a fix

cc/ @ehazlett

[ehazlett]

/cc @diogomonica @cyli

[mdlinville]

Closed via #568

[docker-robott]

Closed issues are locked after 30 days of inactivity.
This helps our team focus on active issues.

If you have found a problem that seems similar to this, please open a new issue.

/lifecycle locked