docs: clarify IOptionsSnapshot<T> performance characteristics

[heissenberg06]

Summary

Added a "Performance considerations" subsection under the
"Use IOptionsSnapshot to read updated data" section to clarify
that IOptionsSnapshot<T> recomputes the full options pipeline
on every scope, regardless of whether configuration has changed,
and documents cheaper alternatives (IOptions<T> and
IOptionsMonitor<T>).

Fixes dotnet#53890