add short version of help, as in quit (#10055)
add short version of help, as in quit (#10055)

* MetaSema: add short version of help, as in quit
Primer: join help and ?, mention .quit and .exit
TRint: mention .?
TApplication: mention .? and CINT 2 Cling

* clarify class name with ROOT7

root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling