Add docker-exec to main manpage

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Add support for copy/add with multiple src files

Part one of solution for issue #6820

Signed-off-by: Doug Davis <dug@us.ibm.com>
Merge pull request #8091 from jfrazelle/exec-main-manpage

Add docker-exec to main manpage
Merge pull request #8076 from duglin/MultiCopyAdd

Add support for copy/add with multiple src files