Add note for fleetsMember reference in ASOManagedCluster

[willie-yao]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
This PR adds a note regarding how to reference a fleet hub from a cluster created under an ASOManagedControlPlane.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #5333

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

NONE

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.43%. Comparing base (87feabd) to head (22b719e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5341      +/-   ##
==========================================
- Coverage   52.43%   52.43%   -0.01%     
==========================================
  Files         272      272              
  Lines       29401    29401              
==========================================
- Hits        15417    15415       -2     
- Misses      13178    13180       +2     
  Partials      806      806              

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[mboersma]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 19decf2cde4a3ff2e5cbfc9eb7ff0e803b68446a

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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