Remove the limitation 'Policies with modifications' in NIM

[vvnginx]

Created few custom policies having 'modifications block' with various 'entityTypes'. Able to apply these policies to datapath instances in NIM. Hence removing this limitation.

Proposed changes

Problem: The limitation ' Policies with modifications' has been present in NIM documentation.

Solution: This limitation needs to be removed from the docs as it is not valid anymore.

Testing: Created few custom policies having 'modifications block' with various 'entityTypes'. Able to apply these policies to datapath instances in NIM. Hence removing this limitation.

If this PR addresses an issue on GitHub, ensure that you link to it here:

Closes #ISSUE

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[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
_{Posted by the CLA Assistant Lite bot.}

[vvnginx]

I have hereby read the F5 CLA and agree to its terms

[vvnginx]

recheck

[ADubhlaoich]

Generally LGTM: made an edit suggestion to replace the production URL link for the feature above the line removed with a Hugo reference, as is our norm.

[mjang]

Approving and merging. @travisamartin I think this is a fairly trivial change. Let me know if I overstepped.