Fix token cache builder return XML documentation

[edbenson]

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Fix token cache builder return XML docs

Description

Corrects the <returns> XML documentation for token cache builder methods.

The affected methods return MicrosoftIdentityAppCallsWebApiAuthenticationBuilder
for chaining, not the service collection.

This is a documentation-only change. No runtime behavior is changed.

Tests are not applicable because this only updates XML documentation.

No linked issue: this is a small documentation correction.