Fix documentation for ReadOnlySequence.GetOffset

[lilinus]

Fixes documentation for ReadOnlySequence.GetOffset to communicate that the returned value is not zero indexed.

Maybe this needs to be changed in https://github.com/dotnet/dotnet-api-docs too?

Related to #75866

[Copilot]

Pull Request Overview

This PR updates the XML documentation for ReadOnlySequence.GetOffset to clarify that its returned offset is not zero-indexed.

• Removed explicit reference to “from the start” in the summary.
• Added a remarks section explaining how to adjust the offset to obtain a zero-indexed value.

Comments suppressed due to low confidence (1)

src/libraries/System.Memory/src/System/Buffers/ReadOnlySequence.cs:533

• The updated remarks clarify the non-zero indexing behavior; however, consider explicitly stating the indexing base or mentioning that this behavior is consistent with related APIs to avoid any ambiguity.

/// The returned offset is not zero indexed from the start.

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[jeffhandley]

Thank you, @lilinus; I'm sorry for the long delay getting to this review.

You are correct that this will also need to be fixed in dotnet-api-docs, but merging here is helpful for our own reference within the codebase and keeps the docs in sync. I submitted dotnet/dotnet-api-docs#11755 for that.