dnsdist: Document that eBPF socket filtering requires CAP_SYS_ADMIN

[rgacogne]

Short description

We used to be able to use only CAP_BPF since kernel 5.8, but the eBPF verifier has been made more strict a few versions later and we now require CAP_SYS_ADMIN again.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 9368950823

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 28 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+1.8%) to 64.606%

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Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdistdist/dnsdist.cc	2	68.19%
pdns/iputils.hh	3	78.39%
pdns/misc.cc	4	63.52%
pdns/recursordist/test-syncres_cc1.cc	5	89.68%
pdns/recursordist/rec-tcp.cc	14	61.36%

Totals
Change from base Build 9368864909:	1.8%
Covered Lines:	124300
Relevant Lines:	161660

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💛 - Coveralls