Update logstash pipeline ID doc desc

[mashhurs]

This PR adds restrictions/acceptance note on Logstash pipeline ID, used through pipeline create REST API.

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?
• If submitting code, have you built your formula locally prior to submission with gradle check?
• If submitting code, is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed.
• If submitting code, have you checked that your submission is for an OS and architecture that we support?
• If you are submitting this code for a class then read our policy for that.

[github-actions]

Documentation preview:

• ✨ Changed pages

[elasticsearchmachine]

Pinging @elastic/core-docs (Team:Docs)

[leemthompo]

LGTM, might want @karenzone to keep me honest though :)

[mashhurs]

FYI: main and 9.x changes - elastic/elasticsearch-specification#5354

[mashhurs]

💔 Some backports could not be created

Status	Branch	Result
❌	main	Conflict resolution was aborted by the user
✅	8.18

Manual backport

To create the backport manually run:

backport --pr 135557

Questions ?

Please refer to the Backport tool documentation

[mashhurs]

Who added these labels?

(Also, good to see you!)

👋 @justzh,
I labelled them but feel free to adjust if it requires.

Also, I am not sure automation didn't backport to 8.18, manually raised PR - #135790