Clarify in the docs matching by wildcard of the sni_domains field in listener components

[vadimeisenbergibm]

title: sni_domains: clarify in the documentation matching by a wildcard

Description:
According to

envoy/source/common/ssl/context_manager_impl.cc

Line 116 in a22159a

const size_t pos = server_name.find('.');

the wildcard is allowed before the first dot, it means that www.example.com will be matched by an SNI domain *.example.com, but not by *.com or *. The proposed PR changes the docs to explain it.

Risk Level: Low

Testing:
Documentation change, no testing.

Docs Changes:
The PR is a docs change only.

Release Notes:
None.

Fixes #3363 .

[vadimeisenbergibm]

Closing it since the commits were not signed, my first PR to Envoy, sorry. Resubmitted with a signed commit #3372.