If one is somewhat new to the command line or shell, it might be
surprising that changes applied to a file like $HOME/.profile will
seemingly not take effect, even if new shells are started.

Add a note about how shells usually only load these when the user logs
into a machine, to minimize the amount of people stuck and confused by
this.

Fixes #24756.

Change-Id: Ic68d8c97933f3f080b151a107633ecad76a163a4
Reviewed-on: https://go-review.googlesource.com/105557Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>