Clarify SMTP password argument

We've confused several users (including me) that `--set
global.smtp.password.secret=smtp-password` should not actually be
the SMTP password. This note tries to drive the point that
`smtp-password` should be left alone.

Relates to https://gitlab.com/charts/gitlab/issues/554
13 jobs for sh-clarify-smtp-password-secret-docs in 17 minutes and 8 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Prepare
passed #235321269
check_docs_internal_links

00:00:46

passed #235321268
danger-review

00:01:01

 
  Review
passed #235321271
review_eks

00:05:12

passed #235321270
review_gke

00:07:47

passed #235321273
allowed to fail manual
stop_review_eks

00:00:36

passed #235321272
allowed to fail manual
stop_review_gke

00:00:39

 
  Specs
failed #235321275
review_specs_eks

00:04:07

failed #235321274
review_specs_gke

00:05:12

 
  Qa
passed #235321277
debug_review

00:01:50

skipped #235321279
allowed to fail
qa_eks
skipped #235321278
allowed to fail
qa_gke
skipped #235321276
review_helm_test
 
  Package
passed #235321280
lint_package

00:00:29

 
Name Stage Failure
failed
review_specs_gke Specs
$ add_license
Task runner pod is gke-review-sh-cla-1aygrc-task-runner-5bc59bf45f-httzg
Unable to use a TTY - input is not a terminal or the right kind of file
WARNING: This version of GitLab depends on gitlab-shell 9.3.0, but you're running Unknown. Please update gitlab-shell.
License added
Uploading artifacts...
WARNING: tmp/capybara: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1
failed
review_specs_eks Specs
$ add_license
Task runner pod is eks-review-sh-cla-rov58o-task-runner-5dcf9f9797-7rtgw
Unable to use a TTY - input is not a terminal or the right kind of file
WARNING: This version of GitLab depends on gitlab-shell 9.3.0, but you're running Unknown. Please update gitlab-shell.
License added
Uploading artifacts...
WARNING: tmp/capybara: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1