docs(api) document header regex in autodocs admin api (#8572)

Co-authored-by: lena-larionova <54370747+lena-larionova@users.noreply.github.com>

autodoc/admin-api/data/admin-api.lua

@@ -984,6 +984,8 @@ return {
             match if present in the request.
             The `Host` header cannot be used with this attribute: hosts should be specified
             using the `hosts` attribute.
+            When `headers` contains only one value and that value starts with
+            the special prefix `~*`, the value is interpreted as a regular expression.
           ]],
           examples = { { ["x-my-header"] = {"foo", "bar"}, ["x-another-header"] = {"bla"} }, nil },
           skip_in_example = true, -- hack so we get HTTP fields in the first example and Stream fields in the second