ports/webcamd: improve pkg-message

[ziaee]

Summary

+ Recommend starting webcamd with sysrc
+ Clarify groups usage

I do not understand step 2, test method:

1. webcamd is listed in rc.conf, but stopped
2. service devd restart does not start webcamd

With suggestions by Alexander88207
See also handbook change: https://reviews.freebsd.org/D46960

Test Plan

make install and pkg inf -D webcamd

[ziaee]

Oops, fix typo