Update triple slash comments on QueryParameterMatchMode and HeaderMatchMode

[Benziza]

Follow-up to #2000, which updated these comments on HeaderMatchMode but left two gaps. Comments only, no behavior change.

1. QueryParameterMatchMode was not covered by #2000

#2000 removed "Only single headers are supported. If there are multiple headers with the same name then the match fails." from HeaderMatchMode, but the equivalent sentence is still on four members of QueryParameterMatchMode:

Only single query parameter name supported. If there are multiple query parameters with the same name then the match fails.

QueryParameterMatcherPolicy iterates over every value bound to the name and matches if any of them matches, so a repeated name does not fail the match. This is already asserted by QueryMatcherPolicyTests:

[InlineData("abc", QueryParameterMatchMode.Exact, true, "a;abc", true)]

which builds ?org-id=a&org-id=abc and expects a match.

The same block also claims the value must match "for each of the respective query string values" on Contains and Prefix, where the matcher is an any-of.

2. NotContains and NotExists drifted again after #2260

HeaderMatchMode.NotContains currently opens with:

The header must exist and the value must be non-empty.

That was accurate when #2000 was written, but the fix for #2260 deliberately made a missing or empty header match, and the comment was not updated. QueryParameterMatchMode.NotContains has the same stale "key must be present" claim.

NotExists says "The header must not exist", while a header that is present with an empty value also matches.

Both behaviors are already pinned by HeaderMatcherPolicyTests:

[InlineData(null, HeaderMatchMode.NotExists, true)]
[InlineData("", HeaderMatchMode.NotExists, true)]   // present but empty -> matches
[InlineData("abc", HeaderMatchMode.NotContains, false, null, true)]
[InlineData("abc", HeaderMatchMode.NotContains, false, "", true)]

Observed on the shipped 2.3.0 package

Route match	Comment predicts	Actual
NotContains on X-Client-Type, header absent	no match	match
NotContains on X-Client-Type, header present but empty	no match	match
NotExists on X-Client-Type, header present but empty	no match	match
Exact on env=prod, request ?env=test&env=prod	no match	match

The wording added here follows the phrasing #2000 introduced for headers.

Verification

build.cmd -projects src\ReverseProxy\Yarp.ReverseProxy.csproj succeeds with 0 warnings and 0 errors, and build.cmd -test -projects test\ReverseProxy.Tests\Yarp.ReverseProxy.Tests.csproj passes on both net8.0 and net9.0.

No test changes were needed: the existing tests already assert the behavior described here.

[Benziza]

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