Slack integration: Highlight channel name examples as such

What does this MR do?

Clarifies the fact that under /services/slack/edit, the place holder text are channel names. Prompted by a customer request (internal ticket).

I need to check whether:

• any tests need updating
• [-] the docs section needs updates now
  • @aqualls: Or can it be removed, seeing that the web UI has pretty much exactly the same lines of explanation below the textfields?
  • Answer below
• [-] the Trigger event for … text could be clarified now
  • Answer below

Hence, no CI run, yet.

Screenshots (strongly suggested)

Before

After

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related issues

• Related to #332281 (closed) where UX work is happening
• Related to #300621 (closed) where I'm working on some UI changes