mesh: shell: Fix & clean up command help
mesh: shell: Fix & clean up command help

Use NULL instead of "", fix typos, and indicate app-key-add last
parameter as optional (to match the command implementation).

X-Original-Commit: c51cad138f1a7663e5fab3d644736fffff0941b3

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble